muscle memory is for example: you get up to a weight of a solid 230lbs. you quit going to the gym, eating right whatever and shrink to say 195lbs... muscle memory is what happens when you start hitting the gym again and quickly blow up to pretty close to that original 230lbs. most likely you'd be able to get to 210-220 fairly easily.

a guy who's never been that large and started at the same time as you going back to the gym would not see the same dramatic size gains that you would. your gains are largely contributed to by "muscle memory"