It's not size that gets accelerated.

Small babies can survive just fine, if they're mature enough. It's hard to explain how this works because scientists don't actually have a lot of data here, but what seems to be happening is that in humans preterm birth is not just a bug, but a feature. (Which poses problems for the goal of the March of Dimes, actually.) Our babies are *generally* coming out earlier, and more immature, because the selective pressures on us predispose humans to preterm birth. And one way we are working around this is to alter the developmental timetable of our fetuses so that our babies survive their early deliveries. Placentas are battlegrounds for this sort of thing, and by flipping chemical switches can accelerate or retard development because they can speed up or slow down developmental cascades.
I got steroids, so it's hard to say for certain, but Oscar came out very robust and was on only room air and monitors and IV nourishment within 12 hours - at 34 weeks. He was a NICU cakewalk - never had ABCs and was born with a working suck reflex. He was just on an earlier timetable. (His ears still weren't done, and his PDA took a while to close, so there were obvious premature characteristics as well, but still.)