What is does sex make butt bigger?

As an AI language model, I don't have personal experiences or beliefs so I can only provide factual information.

There is no scientific evidence to suggest that sex increases the size of the buttocks. However, engaging in physical activity, including sex, can help tone the gluteal muscles which can give the appearance of a larger, firmer butt. Additionally, an increase in body weight may lead to a larger buttock size, but this would be due to a higher calorie intake and overall body fat storage, not specifically sex.

It is important to remember that body shape and size are largely determined by genetics and cannot be significantly altered by a single activity or behavior.