That was very dynamic! Nice entry :)
A couple of moments I'd like to give some feedback on:
- I was not 100% sure what kind of game you want it to be. Should it be a rage platformer? Then the life/gameover system is maybe fine, but then I would wish for more intense sections, as right now there are some parts where you are just running along the tunnel and not that much happens for a while. If it is more of a classical platformer, I think it will be fine without additional punishment and starting from a checkpoint after you fail would be just fine :)
- Maybe it was just my impression, but I think that the game starts quite harsh :D You are immediately expected to make a careful jump, I would maybe prefer that you had time to have some fun, jump around and get acquainted with the terrain.
- I liked the idea of changing your size to proceed but in some cases it seemed to be pretty straightforward, maybe some trickier levels that are not very obvious how to deal with would be fun in that case!