I often think my gf would be happier if I were 'smaller'. I was her first so she doesn't really have a baseline for comparison, but I'm larger than average, especially in girth and we've been trying for about 5 months now and it's still painful for her.
I know it sounds like guys are trying to 'stealth brag' when they talk about this, but nobody wants to hurt someone they care about, and she feels bad about it because she can't enjoy it and therefore I stop. It's starting to put a bit of weight on the relationship because both parties feel bad about it.