I've been squashed a few times by SSBBWs. It's simultaneously amazing and also weird because it's such an intimate thing without the actual intimacy.
I thought having my face sat on would rule, but any time i've tried it, it's like... immediate loss of oxygen. Maybe i need to turn my head to the side instead of having my face directly buried. I may have to try that next time.
There's some... logistical issues. Sometimes these women can't actually get into the positions you want them to. I had Kass squash me about a week before she tragically passed. She was supposedly 640 at that point. She was able to get on top of me face down with her head down by my feet, but couldn't straddle me facing me, or at least she said she couldn't. Some of these women are more flexible and strong than others. It kind of turned into a cuddling session from there, though it honestly fucking ruled. She was so incredibly soft.
Depending on how they're on top of me, if they sit on my sternum area, it's painful and not enjoyable. Might be how i am built. I prefer for them to put the majority of their weight on my lap.
As for how these encounters end, they tend to end with me jerking off in the corner.