I never considered this perspective, thanks for sharing.
So long as big, heavy men don't go on top, I guess you're safe 
Depends on whether they can hold their own bodyweight, and this is often once again not related to actual body weight - some slimmer and very slim men cannot seem to support themselves. Also depends what you mean by safe, if the abdomen is very large the penis is very unlikely to penetrate, if you mean sheer body weight possibly. I have been half suffocated by a moderate belly pushing himself down onto my face, grinding down so hard the bones in my face ached whilst I was underneath in 69.
Also a moderate build person remains the only punter to break a bed of mine - more or less took a running jump at the bed. There was an audible breaking noise "It's not broken" he insisted, that broken piece of wood worked it's way into an awkward position, it was definitely broken although the bed remained useable. Another sat on a sea grass chest not designed for anyone to sit on. Well not unless you are the size of a tortoise or kitten. It could be argued a much larger bodied person might not do the running jump leap, might be more considerate aware of their impact.
Sometimes sex drive disappears in a huge body, similar to that in a very thin or superlean person.
This said I have had some difficult larger body clients, but this was more about their personality, attitude & rudeness. To generalise and lump everyone into a bracket based on bodytype/shape/weight I think is not fair.
