Kind of a hot thread.
There's definitely a lot of extra padding back there - thighs when coming in front, and butt from the back. You can end up going just as deep on a bigger girl, but the fun part is that the extra padding lets you pace things out a little better. With skinny girls it's too easy to just push right in, but if they're thick you can just ease in to it, but your dick's pushing between their plump butt instead of hanging in the cold air.
When you really get going, just push her shoulders down and push in to her hard, and that fat will compress and you can get plenty deep.
Sometimes I wonder if this is part of why so many guys go after very skinny girls, when classically thicker women were more sought after, and big girls look and feel absolutely amazing. If you don't have much length, you would probably just avoid a situation where you couldn't get it in.
Also - pretty hot story JR7.