I think it's just the "what goes up must come down" saying, i think it holds for most things. the universe expands so it must contract, my physics teacher and i got talking about the universe (in a free lesson) and we were talking about what shapes it could possibly take from how the big bang happened, we both believed it was a singularity like in the middle of a black hole. We both came to the conclusion that it would be like a balloon, the rubber being the universe and what we can move in with varied thickness the inner wall being denser material from the big bang which decelerated quicker (possibly from the immense gravity) and the outer part of the wall (i should say surface) being lighter matter which moved thurther.
In a still singulatiy it would be a round universe, but if it had spin the universe wouldn't form round but expand in a flatter shape (like jupiters shape) and with a high enough rotation speed the universe would ultimately form into a doughnut (the ones with the hole in the middle) shape. But all these theorys of how the universe is formed still boils down to the fact that their will be dense pockets of matter which distort space time enough to make the big crunch happen alot earlyer then if the universe formed like a water mist.