Does a Fund's Size Affect How Well It Tracks Its Index?
It’s tempting to assume that a larger fund, with more resources and more scale, should always track its index more precisely than a smaller one. The reality is less tidy: size cuts both ways, easing some frictions while introducing others.
The short answer
Fund size affects tracking error, but not in a single, consistent direction. Very small funds can face proportionally higher trading costs relative to their assets, since fixed costs get spread across a smaller base and small trades may not get the most favorable pricing. Very large funds, on the other hand, can face the opposite problem — their trades are big enough to move prices against themselves, particularly around index reconstitution dates when many other funds are trading the same securities at the same time. There’s no simple rule that bigger funds always track better.
The case for smaller funds struggling
A smaller fund has less flexibility in how it executes trades and less negotiating leverage with counterparties, which can mean somewhat wider effective spreads on the trades it does make. If a fund is too small relative to the number of securities its index contains, it may lean more heavily on sampling or optimization rather than full replication just to keep costs manageable, which introduces its own layer of tracking imprecision. In the smallest funds, a single large investor moving money in or out can also create disproportionate cash flows relative to the fund’s total size, forcing trades that a larger fund could absorb more smoothly.
The case for larger funds struggling
A very large fund holding a substantial share of an index’s total market value can struggle to trade without affecting the price of what it’s buying or selling, an effect known as market impact. This becomes especially visible during index reconstitution, when the fund needs to make sizable trades on a specific date and can’t necessarily spread that activity out to avoid moving the market. Ironically, being the dominant holder of a security can make it harder, not easier, to trade that security cleanly.
Where the middle ground tends to land
Funds that are neither tiny nor dominant relative to their index sometimes have an easier time in practice: large enough to spread fixed costs thinly and negotiate reasonable trading terms, but not so large that their own trading meaningfully moves the market. This isn’t a hard rule — plenty of very large funds track tightly, and plenty of smaller ones do too — but it illustrates why size alone is a poor shortcut for judging tracking quality.
Liquidity of the underlying index matters more
How a fund’s size interacts with tracking error depends heavily on what the fund actually holds. A large fund tracking an index of highly liquid, heavily traded securities may face little friction even at significant scale, while a much smaller fund tracking an index full of thinly traded names can struggle disproportionately. Size matters less on its own than in combination with how easy or hard the fund’s specific holdings are to trade.
The bottom line
Assets under management is a reasonable data point to note, but it isn’t a reliable stand-in for tracking quality by itself. A fund’s actual multi-year tracking record against its stated benchmark remains a more direct and trustworthy measure than any assumption based on how large or small the fund happens to be.