Why small fee differences can matter so much
An expense ratio is a recurring percentage charge taken from fund assets to cover management and operating costs. Because the fee is charged against the invested balance rather than against the original contribution only, it reduces not just current assets but also the future compounding base those assets would otherwise have created.
That is why two funds with similar holdings but different expense ratios can diverge materially over long holding periods. The higher-cost fund has to outperform by enough to offset the fee handicap before the investor ends up in the same place as the lower-cost alternative.