The pledge amount does have an affect on the amount of rewards a pool will receive, but at current levels the difference between minimum and maximum return is not extremely large, and it requires a very large pledge to make a difference. If you are interested, there is a technical article by a member of the community here: https://www.reddit.com/r/cardano/comments/hnm3l6/shelley_incentive_parameters_what_you_need_to/