Not equal slices
When people say “the market,” they often mean a list of large companies, such as the S&P 500, a list of 500 large U.S. companies. An index fund that follows that list does not give each company the same amount of money. A giant firm can be several percent of the fund. A smaller firm on the same list can be a fraction of a percent. You own all of them. You do not own them equally.
What company size means
Size here is the market value of the whole company: the share price multiplied by the number of shares. People call that market capitalization, or market cap for short. A company with a high share price and a huge number of shares is a big slice. A company with a low price, or few shares, is a small slice. The list is rebuilt from those sizes, so the biggest companies sit at the top.
The fund copies that tilt
A fund that follows the list copies those weights. If the ten biggest companies are a large share of the list, they are a large share of your fund. A good year or a bad year at those few names can move the whole fund more than a hundred quiet companies underneath them. Owning “500 companies” is still a wide mix. It is a wide mix with a heavy top.
What a bad year feels like
If those giant companies fall together, the fund falls even when many smaller holdings are fine. That is the same concentration in a different costume. You did not pick the giants one by one. The weighting picked them because they are big. Checking the top holdings of a fund, not only the count of names, tells you what you are actually exposed to.
A fund that holds equal amounts
Some funds give each company on a list the same slice, whatever its size. The smaller companies then matter more, and the giants matter less. Those funds usually trade more often and can cost more, because they have to keep resetting the equal slices as prices move. They are a different product from the usual index fund. Neither one is a complete plan by itself.
What this is not
The heavy top is not a reason to dump the fund and buy only the giants, and it is not proof the fund is a trick. It is how most broad stock funds are built. Two funds with different names can still be the same bet if both are dominated by the same few companies. Read the top holdings once. That page is the real contents label.