Absolutely nothing in Graal changes your hit detection.
The size of your player can change your perception of your movement speed, but your speed does not actually change. I imagine it has a lot to do with your character size relative to world. IE if you're normal, your character is roughly 2 tiles wide. You move .5/tiles a tick, therefor you move 1/4th of your characters size every tick. However if your character size is zoomed 3x, it's now roughly 6 tiles in width. That means every tick, despite moving the same speed your character is moving only 1/12th your characters size every tick. This makes your character appear as though they're moving slower despite nothing actually changing. Same goes in the other direction, smaller morphs like the rat can make it appear as though you move faster even though you do not.