Level design needs more work XD It took me a while and a yolo attempt to figure out how to get out of the first room. I would not have normally thought of forcefully passing through the yellow wall to fall past the red floor on the bottom. I'm assuming that's the intended solution since it requires you to be the tiniest size to go through the bottom, and all of my other attempts to jump over the red were unsuccessful. With the platform pushed right up against the yellow wall like that, I don't think a lot of players would think that there actually is a gap between the platform and the real grey wall.
The choco chip in the second room is unobtainable? I assumed I would need to fall in from another room to get it since I see a gap above it, but didn't get far enough to figure out of that's actually the case. I only got as far as the 3rd room because of how unforgiving the red floor is. Resetting the player back to the start of the game just feels unnecessary. I'm assuming the intent is yellow reduces size by 1, then resets player if size is too small, while red kills you instantly. Despite it being a kill though, having to go through the whole game again makes it way too tedious and discouraging. Would have though that it being a reset to checkpoint is enough, since the player isn't easily blasting through the levels normally.
I'll make another guess again that the red resets the whole game because it starts to feel exactly the same as yellow? Since I was staying at the tiniest size for quite a while, yellow will just keep resetting me anyways, and would feel the same as red if red only does a checkpoint reset. Which brings in the point that the size mechanics could be utilized a lot more within levels, making it feel a little bit more like a puzzle platformer. More choco chips could be placed around, and players will then have to figure out when to enlarge with which choco chip, and when to touch the yellow to reduce size to pass through which gap. That would differentiate yellow and red areas by a lot (ie. yellow is to be utilized vs red is to be avoided at all costs), and make playing the game more compelling as you're trying to figure out how to get to the next level.
I think there's a lot to be expanded on and explored here, it could make for a silly adventure as a cookie, and use inspiration for obstacles depending on the setting. I hope my feedback helps :) Music choice is great btw!