By "ON TOP" I assume you are talking about having sex with you on top.
I can assure you that's my favorite position, for many reasons:
1 - I have read that the **** is stimilated more
2 - I have read that the penis goes in deeper
3 - I have read, and it's been my experience that the woman can go much slower, so it's much more pleasurable, and she seems to have a better time, as do I
4 - I love being able to play with her breasts in that position, and I love the view
I also love oral in that position - I Love to lick my g/f when she is "on top"
As far as being a little heavy, and what that has to do with the position, are you worried about the weight pressing down ?
It has never bothered me - sometimes my partner does lean heavy on my chest - but then she doesn't press so hard -
If she was to press hard on my chest the whole time, I would have to tell her to let up a little, but that has never happenned.
there seems to be two ways of interpreting your question. In either case, it doesn't bother me at all. In the second interpretation, you did say "..a little..." not "grossly" and I'm assuming you aren't killing your chap (if you're a woman) once you've saddled up. On top can mean several positions, but none have ever bothered me. I haven't been with hugely overweight women, but some 'big boned' ones and its all very enjoyable.
Doesn't bother me one bit. To be honest I really don't think about it much. I'm so lost in the moment that I don't think about such things. As far as being on top, I love for her to be on top. Some women say that they get more stimulation in that position and I love looking at her breasts when she is on top.