Baby rabbits are just as big as a squirrel and if it ate more than one, then that would account for the length of the bulge. From looking at the pic and not really having any idea of size, it just looks a bit too long to be a squirrel, then again I don't have much to reference the size.
We don't have Jack rabbits around here, baby rabbits can run away before they would be anywhere near as big as what he ate. For reference, those are mostly maple leaves. I wanted to put something down next to him as he was not going anywhere fast, but did not want to stress him out.
I've seen my cats drag in whole litters of baby rabbits, I've watched a black snake eat an entire litter before, what happens is the baby's reaction a lot of times is just to freeze. Depending on age, they don't like to leave the nest even when a predator is right there. That being said, I am not ruling out a squirrel, but it's difficult for snakes to get squirrels unless they surprise them. May of very well happened just offering an alternative Heck another alternative is that it could of found a chicken coup and ate a few eggs . Use to happen all the time when my grandma's neighbor had chickens. See a big ole fat snake slinking out of the coup lol. Only really way to tell is to catch it and see what it had for dinner lol.