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.
hope that helps - love you question