Does a Fund's Size (AUM) Matter When Comparing Funds?
A fund’s total assets under management, often shortened to AUM, gets mentioned in passing on most fact sheets, but it’s easy to either overrate or ignore entirely when comparing options.
The short answer
Fund size can matter in a few concrete ways: larger funds often benefit from economies of scale that can support lower fees, and they tend to have deeper trading liquidity. But size can also work against a fund when its strategy involves less liquid or narrower markets, since a very large pool of assets can become harder to move without affecting prices. Size is a relevant data point, not a simple better-or-worse signal.
How scale can work in an investor’s favor
Running a fund involves relatively fixed costs — administration, compliance, trading infrastructure — spread across all the assets in the fund. As a fund grows, those fixed costs get divided among more assets, which can support a lower expense ratio over time. Larger funds, particularly exchange-traded funds, also tend to trade with tighter bid-ask spreads and higher trading volume, which can reduce the practical cost of buying and selling shares.
Where size can become a constraint
For funds that invest in broad, highly liquid markets, size is rarely a meaningful limitation. But for funds pursuing a narrower strategy — small-cap stocks, a specific sector, or less liquid asset types — a very large pool of assets can be harder to invest without the fund’s own trading affecting the price of what it’s buying or selling. This is sometimes described as a capacity constraint, and it’s one reason a strategy that performed well at a smaller size doesn’t always perform the same way once it’s grown substantially.
Small funds carry their own considerations
A small or newly launched fund isn’t automatically risky, but it comes with different things to weigh: less operating history to evaluate, potentially higher relative costs if assets haven’t reached a scale that supports lower fees, and in some cases lower trading volume that can widen spreads. None of this makes a small fund a poor choice by default — it simply means size is one more variable worth factoring in alongside cost, strategy, and how closely it tracks its stated benchmark.
Questions worth asking about size
- Does the strategy depend on a liquid, broad market, or a narrower one? Size matters far more in the latter case.
- Has the fund grown or shrunk substantially in recent years? A rapidly growing fund pursuing a narrow strategy is worth watching for capacity effects.
- Is the expense ratio in line with funds of similar size and strategy? If not, it’s worth understanding why.
The takeaway
Fund size isn’t a shortcut for judging quality on its own. It’s most useful as context: a signal about likely liquidity, potential economies of scale, and, for narrower strategies, possible capacity constraints. Weighing it alongside cost, strategy, and performance history gives a more complete picture than looking at AUM in isolation.