It's really easy to forget how heavy people are; I'm not commenting on obesity or anything, just how much even an average-weight person actually weighs. The extent to which people commonly feel most of a person's weight on them would be either during sex or in a situation like wrestling. In both these instances, you have most of one body supporting most of another, so it's much more spread out and all your muscles can work to support or resist it. When you have a man standing with his full or near-full weight standing on your chest, you suddenly find yourself with all that weight concentrated in a very small area. What's more is your brain intuitively understands this and knows that fighting with the same fervor when you had your entire body to work with is likely to cause injury, so you almost naturally suppress fighting it. When a good bit of that weight shifts to the other foot which is cutting off your windpipe, you also realize that no matter how hard you grab onto his ankle and try to lift it, from your current position you're not stopping the onslaught.
Basically, you're fucked.