If the medium rat leaves a slight bulge in him, it's a good size for him and he doesn't need to be moved onto larger prey. Yes, snakes can eat prey that is considerably larger than they are, but in the wild you have to remember they don't always have regular food sources, so it'd be like you not eating for three days and then stuffing your face with a four-patty hamburger and fries. Sure, you can eat it, but smaller meals more often is easier on the digestive system and seems to keep them from going off of feed much more.