Fund fees are quoted as small annual percentages, which makes them look trivial against market movements. The mechanism by which they are charged is what makes them significant over long periods.
The fee is charged on assets, not returns
An expense ratio is deducted from the fund's assets continuously, whether the fund gained, lost or stood still that year.
This differs from a performance fee, which is contingent. An asset-based charge is a certainty applied against an uncertain return.
In a year when markets fall, the fee still applies to the reduced balance, which means costs are being paid out of a shrinking base.
Why the effect grows rather than staying flat
A fee removes money that would otherwise have remained invested and earned returns of its own, so each year's charge also removes all the future growth that amount would have produced.
The loss therefore compounds alongside the portfolio, and the gap between a low-cost and a high-cost fund widens rather than staying proportional to the fee difference.
Over a working lifetime the cumulative effect of a modest annual difference can amount to a substantial share of the final balance, which is why the comparison matters most for long horizons.
The largest balance pays the largest fee
Because the charge is proportional to assets, the absolute amount paid rises every year that the portfolio grows.
The heaviest charges therefore fall in the final years before the money is needed, which is exactly when the balance is largest and the time available to recover is shortest.
This is the opposite pattern from most costs, which tend to be largest at the start, and it is why fees are often underestimated by investors who assessed them when balances were small.
What the headline figure leaves out
The quoted ratio covers management and administration but generally excludes the fund's own trading costs, which are incurred inside the portfolio and reduce returns without appearing as a fee.
Bid-offer spreads on buying and selling the fund itself, platform charges and any advice fee sit outside the ratio as well.
The total cost of ownership therefore exceeds the number most often compared, and the components that are excluded vary between products and jurisdictions.
Why cost is the one input that is known
Future returns are uncertain, and no assessment of a manager's skill removes that uncertainty. Costs are disclosed in advance and are almost entirely predictable.
That asymmetry is why cost receives attention disproportionate to its size. It is the variable an investor controls directly rather than one they are forecasting.
A higher fee is not automatically worse, but it has to be justified by something the cheaper alternative does not provide, and the burden of that comparison sits with the more expensive product.