Trying to figure out why her lapdancing hurts

I know that there is a thread already started about painful lapdances but mines has a little different "spin" to it and that thread is already full. There is this dancer named Babygirl whom is a favorite of mine(or useto be a fav). She is a very sexy blackgirl with a athletic build about 5'6 & 140 to 145lbs of toned muscle. The thing is that when she giving me a lapdance and sits in my lap she feels like " a ton of bricks" dropped on my lap. On the other hand, there is this other dancer named Libby who is kind of overweight and not in the shape that Babygirl is in but she feels quite soft and not like a ton of bricks that babygirl feels like. Why is this? Libby is about 20lbs heavier than Babygirl but doesn't feel as heavy or painful sitting in my lap as babygirl? BTW I would like to say I am glad to be back on the messageboard. I had some problems with my computer for the past month or so that I had to get straighten out.