The question of when an ETF is too big was asked by market observers and journalists when the Vanguard S&P 500 ETF (VOO) reached more than $1.0 trn in assets under management (AUM) at the beginning of June 2026. But is this a valid question, or just an attempt to gather attention?
Well, when the assets under management of a single ETF reach such a high level, this question is obviously a valid question. However, the truth of the intention of the person asking this question can be found in the answer, if they provide one.
From my point of view, the answer is quite simple—an ETF (or a mutual fund) becomes too big when it becomes the market, meaning when transactions of the ETF or mutual fund impact the market in both directions. A second criterion is the liquidity of the single positions. If the ETF/fund can easily buy, but even more importantly, liquidate any position in the portfolio over a reasonable period of time (to avoid too much market impact), it is not too big for its market.
Now, let’s talk about the Vanguard S&P 500 ETF. This ETF is investing in the largest and most liquid stock market on earth. When it comes to this, I don’t believe that there is a general liquidity risk related to VOO which is caused by its $1.0 trn-plus in assets under management. That said, an ETF/fund of this size may move the market down if there is a fire sale in shares of VOO. But will this be a single event which only happens to this ETF? Most likely not, if U.S. investors start to reduce their exposure to the S&P 500 this will impact literally all stocks in the index, as well as all ETFs/funds which are tracking the index. That said, one needs to bear in mind that the second (iShares Core S&P 500 ETF (IVV) – $854.9 bn in AUM at the end of May 2026) and third (State Street S&P 500 ETF Trust (SPY) – $783.8 bn in AUM at the end of May 2026) largest ETFs in the world are on the way to hit the $1.0 trn mark. Hence, it is most likely that a fire sale would also hit other products and not only VOO. If there is more than one product involved in a fire sale it doesn’t mean that this will reduce the pressure on the market, but it may reduce the pressure on a single product.
Graph 1: Assets Under Management of the 3 Largest ETFs Globally – January 1, 2016 – May 31, 2026 (in mn USD)
Source: LSEG Lipper
That said, I am pretty sure that all asset managers, who act in the best interests of their investors, have their own liquidity measures in place and are monitoring all kinds of risk factors within their risk management departments.
At the end of the day, such a market environment would be uncharted territory, and only time will tell what really happens during such an event. Nevertheless, one thing that can be said is ETFs have so far always been liquid and acted as expected during rough market conditions.
To be clear, the money invested in the Vanguard S&P 500 ETF and other products tracking the S&P 500 is money invested in the broad U.S. stock market. This means the money would most likely be invested in the market anyway and would cause (at least) the same pressure on the market if the respective stocks were bought or sold via a different vehicle or single brokers.
I am not saying that one shouldn’t be concerned if so much money is invested in a single ETF/fund, but there is no need to be frightened by this as long as the size of the product corresponds with the size of the underlying market. This means that the question of whether an ETF or mutual fund is too big when reaching a given size is a valid question. The answer, however, should not be used as clickbait or a reason to raise concerns over a problem which doesn’t exist or is much smaller than the answer of the author to its own question suggests.
The views expressed are the views of the author, not necessarily those of LSEG.
This article is for information purposes only and does not constitute any investment advice.
