The QQQ index rules are changing next month. There are also some big IPOs planned this year (some big AI companies), and the new rules will make these AI IPOs quickly show up in QQQ. In some cases, the float will be extremely low (under 20%). The FT Alphaville article summarized the changes as "Engineered scarcity, meet dumb money".

QQQ has 47% of Assets in Top 10 Holdings.
SPY has 37% of Assets in Top 10 Holdings.

ETF AUM
SPY $700B
QQQ $400B
TQQQ $27B
QQQE $1B

Strategy (MSTR), a leveraged bitcoin investment vehicle, is an example of different index methodologies. QQQ usually excludes financial companies, but included MSTR (calling it a technology stock). In contrast, the S&P 500 committee excluded MSTR from SPY.

Most of the Nasdaq 100 stocks are now in the S&P 500. There was a rapid increase between 1995 and 2010.

atYearEnd  N1 in S5
  1985        20
  1990        26
  1995        27
  2000        53
  2005        54
  2010        75
  2015        83
  2020        79
  2025        88

=== links ===

gtr1.net: GTR1 Backtester - S5NasOverlap

Nasdaq-100 Index Consultation – February 2026
"Please note that any resulting changes to the methodologies will be implemented after the
March 2026 Quarterly Rebalance. ...
1. ... Nasdaq proposes to consider a company’s entire market capitalization (both listed and unlisted shares) for the purposes of ranking and inclusion, while continuing to consider only eligible listed share classes for the purpose of weight determination. ...
2. ... Nasdaq proposes to incorporate a “Fast Entry” rule. Under this rule, a new Nasdaq listed company will be evaluated for index inclusion. If its entire market capitalization ranks within the top 40 current constituents2 in the index, it will be announced as a “Fast Entry” addition to the index with at least five trading days’ notice and would be added to the index after fifteen trading days. ...
3. ... Nasdaq proposes a new approach for including and weighting low-float securities (those below 20% free float). Each low-float security’s weight will be adjusted to five times its free float percentage, capped at 100%. ... "
indexes.nasdaqomx.com - NDX Consultation February

Nasdaq wants to fast-track founders and let index trackers hold the bag, Feb 26 2026
This article is behind a paywall, but you can get a free subscription to gain access.
"Nasdaq is proposing changes to its flagship Nasdaq 100."
Financial Times: Content Cdf3cbb8 a2b5 439e b8a8 691782cd763b