I'm no physician but I'm thinking it has more to do wind resistance than anything.

The heavier you are, the more wind resistance would be needed to hold you back. Unless you are morbidly obese, your frontal area change isn't going to be drastic enough to make you equal with the light riders.

Noone will be hitting terminal velocity...so as fast as you think you're going, you can go faster.

??? Does anyone here has a physics degree? I think we all need a refresher.