The most popular index funds are weighted by market cap: the bigger the company, the more stock in it the fund holds. Or in other words, the holdings are sort of "normalized" with respect to the fraction of the total market each company represents, such that the performance of the fund matches the performance of the market.
There's no real agenda there.
(There are other sorts of index funds, such as "equal weight" funds that invest the same amount in every company in the index, but if you think about it, that's weird because it's basically skewing its investing to the smallest companies.)