Here is a puzzle from two filings. IWM and VTWO track the same index, the Russell 2000. IWM's filing shows a fund that behaves like 840 equal holdings, its largest stock 0.38%. VTWO's shows 514, largest stock 1.83%, Bloom Energy. Same index, same two thousand names, a difference of more than 300 in the number that describes the portfolio. Nobody made a mistake. The two filings are one month apart, and the month in between is June — when the Russell indexes are rebuilt.
That is the whole essay, and the tables below show it happening every year since 2019. IJR, the other large small-cap fund, is on a different index with a different rule and does not do it at all.
The figures
- IWM (iShares Russell 2000), period 2026-06-30: 2,016 stocks, $83 billion, largest position 0.38%, top ten 3.4%; behaves like 840 equal holdings once 17% of securities-lending collateral is set aside.
- VTWO (Vanguard Russell 2000), period 2026-05-31: the same index one month earlier — 1,954 stocks, largest position 1.83% (Bloom Energy), top ten 7.8%; behaves like 514.
- IJR (iShares Core S&P Small-Cap), period 2026-06-30: 655 stocks, $111 billion, top ten 7.5%; behaves like 356.
- The IWM–VTWO gap is the calendar, not the funds. The Russell indexes rebuild once a year at the end of June. In the eleven months between, the small-caps that rise stay in the index and grow to 1–2% weights; on reconstitution day they graduate out and the index resets to 2,000 near-equal names. IWM's effective count has done this every year on record: 666 in March 2026 → 840 in June; 764 → 816 in 2025; 623 → 835 in 2024. VTWO's May filing caught the index at its most concentrated; IWM's June filing caught it a day after the reset.
- IJR does not have a June. The S&P 600 is maintained by a committee and rebalanced quarterly, so its effective count has stayed between 353 and 393 for the same two years.
- IWM and IJR overlap by 46.9% of weight (540 shared companies); held together they behave like 660, fewer than IWM alone. IWM is 93.7% US-domiciled by filed country, with Canada 1.4%, Bermuda 1.1%, Cayman 0.7%.
- This describes filings on dates. It is not a view on small-caps or on which fund to hold.
Data as of 2026-06-30 (IWM, IJR) and 2026-05-31 (VTWO) · received by the SEC 2026-08-25, 2026-08-27 and 2026-07-29 · written 2026-09-24. Re-measured as the next filings arrive.
Three funds, one month apart
Source: each fund's Form N-PORT, rows in the point-in-time files. Weights are shares of each fund's own investments after securities-lending cash collateral is removed; the collateral note explains why that matters for IWM in particular.
| IWM | VTWO | IJR | |
|---|---|---|---|
| Index | Russell 2000 | Russell 2000 | S&P SmallCap 600 |
| Filing period | 2026-06-30 | 2026-05-31 | 2026-06-30 |
| Stocks | 2,016 | 1,954 | 655 |
| Net assets | $83B | $18B | $111B |
| Largest position | 0.38% | 1.83% | 0.68% (largest stock) |
| Top ten | 3.4% | 7.8% | 7.5% |
| Behaves like | 840 | 514 | 356 |
| Cash collateral on loan | 17.0% | 2.2% | 5.5% |
| US-domiciled | 93.7% | 100.0% | 97.0% |
VTWO is the same index as IWM, and on the day IWM filed it would have shown the same numbers. The month between the two filings contains the reconstitution.
The June reset, in the filings
| Filing period | IWM behaves like | IWM largest | VTWO behaves like | VTWO largest | |
|---|---|---|---|---|---|
| 2024-03 / 02 | 623 | 1.92% | 701 | 1.56% | |
| 2024-06 / 05 | 637 | 1.63% | 656 | 1.54% | |
| 2024-09 / 08 | 835 | 0.51% | 849 | 0.48% | |
| 2024-12 / 11 | 819 | 0.55% | 805 | 0.59% | |
| 2025-03 / 02 | 764 | 0.64% | 773 | 0.58% | |
| 2025-06 / 05 | 816 | 0.53% | 734 | 0.71% | |
| 2025-09 / 08 | 728 | 0.76% | 795 | 0.66% | |
| 2025-12 / 11 | 717 | 0.76% | 695 | 0.91% | |
| 2026-03 / 02 | 666 | 1.01% | 653 | 1.12% | |
| 2026-06 / 05 | 840 | 0.38% | 514 | 1.83% |
Read the two funds together and the sawtooth is the same: from each September the effective count falls and the largest position grows, through the winter and spring, until the June rebuild. IWM's June 2026 filing (period end 30 June, after the late-June reconstitution) is the top of the tooth; VTWO's May filing is the bottom. In 2024 the pattern is identical with the roles reversed by the calendar: VTWO's August filing (849) and IWM's September (835) both sit just after the reset.
Two consequences for anyone using these numbers. A Russell 2000 fund's concentration depends on the month you measure it, by a factor approaching two; and the "1,954 stocks" or "2,016 stocks" on the fact sheet is stable while the number that describes the portfolio swings between about 500 and 850. This site reports both and dates them.
The reset happens every June. The level it resets to is falling.
The sawtooth is the short story. The long one is where the teeth are landing. Every June filing IWM has on record, back to 2020:
| June filing | Behaves like | Top ten |
|---|---|---|
| 2020-06-30 | 899 | 3.19% |
| 2021-06-30 | 990 | 3.12% |
| 2022-06-30 | 986 | 2.71% |
| 2023-06-30 | 927 | 2.93% |
| 2024-06-30 | 637 | 5.81% |
| 2025-06-30 | 816 | 3.70% |
| 2026-06-30 | 840 | 3.42% |
At the top of the 2021 tooth the fund behaved like 990 equal holdings out of 2,002. At the top of the 2026 tooth, 840 out of 2,016. The stock count barely moved; the spread did. A reconstituted Russell 2000 in 2026 is a more concentrated thing than a reconstituted Russell 2000 in 2021, and the index is doing that on its own — the fund simply holds what the index says.
The 2024-06 reading of 637 is the exception and worth naming rather than smoothing: that filing period ends the day the reconstitution takes effect, so it caught the index before the reset rather than after. The following filing, 2024-09, reads 835. Two adjacent filings, 637 and 835, on a fund whose holdings list barely changed in name count. This is why the date matters more than the count.
The reconstitution is visible in what was bought
Concentration is one way to see the rebuild. The position list is another, and it is blunter. Comparing each fund's latest filing with the one before it:
| Period compared | Positions added | Dropped | Turnover proxy | |
|---|---|---|---|---|
| IWM (contains June) | 2026-03-31 → 2026-06-30 | 246 | 175 | 20.9% |
| VTWO (ends before June) | 2026-02-28 → 2026-05-31 | 41 | 56 | 11.7% |
| IJR (quarterly committee) | 2026-03-31 → 2026-06-30 | 33 | 36 | 14.5% |
Two funds, one index. The quarter that contains the reconstitution took on 246 new companies; the quarter that ends a month short of it took on 41. Six times the intake, from the same index, because of where the calendar cut. IJR, rebalanced by committee through the year, did 33 — it never has a June because it never stores one up.
Turnover here is half the sum of all weight changes, a proxy rather than the fund's reported turnover, and positions are matched on identifiers. Every filing pair, per fund.
One index, two very different second businesses
IWM and VTWO hold the same companies. What they do with them is not the same at all, and the filings say so in a part nobody reads — Item B.4.a, where a fund names the firms borrowing its stock.
| On loan, % of fund | Borrowers named | Largest borrower's share | |
|---|---|---|---|
| IWM | 16.03% | 30 | 82.3% — JP Morgan Securities |
| VTWO | 2.08% | 29 | 21.7% — BofA Securities |
| IJR | 5.41% | 30 | 15.4% — JP Morgan Securities |
IWM lends roughly eight times as much of itself as VTWO does, and four fifths of that sits with a single counterparty. VTWO, on the identical index, lends a fiftieth of its assets and spreads it. Neither is a verdict — lending is disclosed, collateralised and ordinary, and the cash collateral IWM reports is 106% of what it lent. But two funds tracking one index are not the same instrument, and this is one of the places the difference lives. How that concentration got to 82%, quarter by quarter.
What the three actually returned, as they reported it
Not from a price feed: these are the monthly total returns each fund filed in Item B.5 of its own N-PORT, compounded.
| Last 12 months filed | Whole series | Months | Best month | Worst month | |
|---|---|---|---|---|---|
| IWM | +40.6% | +110.5% | 84 | +18.4% | −21.7% |
| VTWO | +43.2% | +114.5% | 81 | +18.5% | −21.7% |
| IJR | +37.5% | +111.1% | 84 | +18.2% | −22.4% |
Seven years, three different small-cap index methodologies — a June-rebuilt index of 2,000, the same index at a different sponsor, and a committee-run index of 600 filtered for profitability — and the cumulative figures land within four points of each other: 110.5%, 114.5%, 111.1%. The worst months are within a point too.
All three agree with a separately computed price series, so the figures are published rather than withheld; the funds where this site's two readings disagree are named here. Past returns say nothing about future ones, and the point of putting them here is not to rank the three — it is that the concentration differences above, which are large and real, did not produce return differences of the same size.
IJR: the same size band, a different rule
| Filing period | IJR behaves like | Largest stock |
|---|---|---|
| 2024-03 | 367 | 2.1% |
| 2024-09 | 385 | 1.6% |
| 2025-03 | 382 | 1.6% |
| 2025-09 | 353 | 1.7% |
| 2026-03 | 393 | 1.4% |
| 2026-06 | 356 | 0.7% |
The S&P SmallCap 600 is a third the size of the Russell 2000 and is changed by a committee through the year rather than rebuilt on a date, so its effective count moves within a narrow band. The fund's largest filed line is a BlackRock cash fund at 2.1%, which is liquidity rather than collateral and is counted; the largest stock is FormFactor at 0.7%.
IWM and IJR together
| Held | Behaves like |
|---|---|
| IWM alone | 840 |
| IJR alone | 356 |
| IWM + IJR, equal size | 660 |
Overlap by weight 46.9%, 540 shared companies. The S&P 600 is largely a subset of the Russell 2000's universe filtered for profitability, so half of IJR's weight is in companies IWM also holds; the other half is the effect of the filter. Held together the pair behaves like 660 — more than IJR, fewer than IWM. Two small-cap index funds are one small-cap position with a profitability screen applied to half of it.
Where the Russell 2000 is domiciled
93.7% of IWM's investments are filed as US-domiciled. The rest: Canada 1.4%, Bermuda 1.1%, Cayman Islands 0.7%, United Kingdom 0.5%, Ireland 0.4%, Israel 0.4%, Marshall Islands 0.4%, and eleven smaller codes. The offshore domiciles are mostly US-listed insurers, shippers and holding companies incorporated abroad; the index is a US index and the filing's country field records incorporation, not business. VTWO files 99.98% US for the same index — the two sponsors report the same companies' countries differently, as this site has found elsewhere.
What this can and cannot tell you
- As of the filing dates above. For a Russell 2000 fund the date is half the number; a March figure and a September figure are not comparable without saying so.
- Effective holdings measures weights, not risk; 840 small companies and 356 small companies are both small-cap exposure, and this measure cannot rank them.
- Overlap pairs IWM and IJR at the same period end (2026-06-30).
- Nothing here is a view on small-caps, on the Russell versus S&P methodology, or on holding any of the three.
Data: IWM · VTWO · IJR · IWM vs IJR · position files IWM, VTWO, IJR.