Does fish oil pills really make your butt bigger?

Not if proper doses. Fish oil is oil, which is a fat, so has some calories. Taking 1-4 grams of fish oil is the same as eating 1-4 grams of fat, which is not much compared to average american diet. One egg yolk is about 4 grams of fat. If a person is taking fish oil, he should also cut back on other fats. If one's butt keeps getting bigger, the likely cause will be other foods, not the fish oil pills.
Normally. weight gain does not effect one specific part of the body alone. If one took in a great deal of oil (which is heavy in calories) it could lead to weight gain. But there is no way no know where the weight would collect.