Sometimes when that sort of thing happens I just push back, literally. (Another piece of the passive-aggressive approach.) There comes a point when you just don't give a s*** anymore whether your knee and his knee are touching, or whatever--if it's happening anyway, you might as well be as comfortable as possible and not give yourself a backache trying to shrink yourself to half your normal size. So you spread out as far as possible too. You may find it works wonders. Just go ahead and shove.
The problem is that women are raised to be too accommodating. For once, DON'T be, and you'll be amazed at how satisfying it is.