I'm a chubby chaser so I love big guys (I'm talking 250lbs plus). I'm also only about 5'7 and skinny so that means that there's a big size diference between me and the guys I'm into.
My BF is about 300lbs I'd say (we don't have scales at the moment) and he's about 6'2 so he towers over me and I can only just get my arms around his waist.
I used to love topping him, his arse is so round and plump and he has lots of soft warm fat to grope as I ploughed him. I say "used to" because recently he's decided that he prefers topping and that I should be his bottom bitch full time (which I'm really enjoying).
Big guys make great tops. They can use their weight to pin you down or drive themselves deeper into your greedy hole.
The only problem is that they often have quite small dicks, but that isn't a problem for me. I prefer smaller dicks because I'm a small guy so they feel more comfortable inside me, and because my BF's cock is small that means it's really easy for him to slide it up into my breeding hole whenever he wants. We don't use lube at all any more, he precums enough to naturally lube me up
Size difference in all aspects is a big turn on for me!