I don't see the problem with verbal stuff, either humiliation for the smaller guys or praise for the really big guys, or vice versa for that matter, if that's what turns them on. I'd never do it unless asked to, and I'd never do it if I thought it was in any way hurtful to the guy asking for it. Aside from that, I can't see how it's any different than any other kind of sexy/dirty talk you could do during sex.