Not necessarily. Sometimes if they are skinnier in size but still bigger by weight that can mess up that amount. ;]
Take it by the rule of thumb of the mouse (or rat) size being about the same size as the THICKEST point on the snake. If the snake is a bit skinny, you can go down to a bit smaller than that.
It's not necessary to overfeed him, he will eat bigger mice than corn snakes at a younger age as corns don't get nearly as much diameter on their body as ball pythons.
My 120g year old male ball python is about twice as thick as my 5 year old female corn snake! :]