I love it. really great fit for the theme,
Cool that each size of the player has different skills, fire frequency, melee and ranged differences.
only things i would change is to not spill blood when attacking an enemy which i can currently not hurt because i am not matching his scale.
Also as Dusty mentioned, maybe increase the difficulty curve a little bit, on normal it is maybe a little bit slow.
And give the enemys maybe slightly less HP, to be not such bullet sponges sometimes.
Maybe you can try to map the 1,2,3 scale change to the mousewheel or something.
Maybe this would be cool for a couch coop game with a buddy and two controllers?
Very well done!
Respect ;)