Nice, well polished work!
Definitely a unique take on the theme, I don't image ill be seeing anything else like this within the other games, and I think this is a harder jam to think of some out of the box ideas.
Having each size play roles in and outside of the players size is interesting, where the skills can be useful to use even if you aren't in their specific size, like the various dashes when needed. I do like how the sizes also have semi different style gameplay, but I do wish there was a bit more disparity between small and medium, since they are essentially the same. The only main difference I found with medium is that you don't really have to be in it for that long, you can just switch by it and throw a dynamite. The one size that felt like it needed to be in a lot was the large one, so maybe the design was that it would be more difficult to get closer to enemies when you weren't using the small and medium sizes as much.
The boss was a great change of pace, and added more complexity. I would like to see some more enemy times or variety requiring players to adjust and think about their size more. Maybe an enemy that gets smaller as you hit it, requiring to be killed in all 3 sizes. Or just general more attacks other than proximity, like having some shooting projectiles. You could have the large size be able to deflect them too!
All of the visuals were super clean, works well with the simple style, good use of squash and stretch to get a lot of animation out of having no actual animations.
My only nitpick would be that the music doesn't loop, but finding good looping music is difficult.
Good job!