I know because I am fat. I am usually the fattest woman at the pool but I don't let that stop me. If you don't want to look at me then exercise your neck and turn your head!
My problem is that most of my weight is concentrated in my butt and thighs! That means that my ass wants to float to the surface while my lighter upper body is forced forward by the force of my butt hovering on top of the water.
It's too bad I can't breathe through the blow hole in my rear - I could stay in the water forever!