A fund fee looks small when written as a fraction of a percent, and large when written as a dollar total over decades. Both figures describe the same charge.
The fee is levied on assets, not on gains
An expense ratio is applied to the value of what is held, and it is charged whether the year was good, flat or severely negative for the fund.
That is different from a performance fee, which takes a share of profits only. The asset-based charge is far more common in retail funds and far more predictable.
Because the base is the balance rather than the profit, the dollar amount surrendered rises every time the account grows, even though the stated rate has not moved at all.
What is removed cannot compound afterwards
The visible cost of a fee is the money taken this year. The larger cost is the return that money would have earned in every year that followed.
Each deduction removes a small piece of the base that future growth is calculated on. The shortfall widens over time because it is being multiplied rather than merely repeated.
This is why a fee difference that looks trivial across one year becomes substantial across thirty. The mechanism is the same compounding that makes long holding periods attractive in the first place.
Small rate differences separate slowly then quickly
Two funds tracking the same index with slightly different charges will look nearly identical for several years. The divergence is invisible at first because the base is still small.
As the balance builds, the gap between them widens in absolute terms each year. The separation is barely noticeable early and hard to ignore by the end of a working life.
Comparing funds on cost is therefore most valuable at the start, when the decision has the longest runway. Switching later captures only the remaining years of the difference.
The stated ratio does not capture every cost
Trading costs inside the fund sit outside the expense ratio in most disclosure regimes. Commissions, spreads and market impact from portfolio turnover are paid from assets but reported separately.
A fund with a low headline charge and heavy turnover can be more expensive in practice than one with a slightly higher ratio and a patient portfolio.
Tracking difference against a benchmark captures both effects at once, which is why it is a more complete comparison than the headline number by itself.
Why fees have fallen and what that changed
Competition among index providers pushed headline charges down sharply over the past two decades, to the point where cost is no longer the main differentiator among large index funds.
The competition has shifted to securities lending revenue, tax efficiency and the quality of trade execution. These are less visible than a published ratio and matter about as much.
For funds that are already inexpensive, the remaining decisions are about structure and behaviour rather than price. Cost still matters most where charges remain high and the holding period is long.