Very cool idea that can be greatly expanded upon! Managing your ammo to change your speed AND interact with various “scale pads” depending on your weight could be a great mechanic! The pads can offer buffs/debuffs depending on your weight and you can choose to step on them to try your luck. You could also make it so that the player could collect additional stones in order to increase their weight if necessary.
There can also be a mechanic where you “pay” with ammo to open certain chests that give you different power ups etc. and instead of collecting additional ammo, maybe the ammo can regenerate slowly in time.
Good luck!