Nice game. I like the idea of using your size as a bullet.
I think the tree health bar should be more visible. At the end of the game, I spotted it.
Maybe pointing it in the tutorial screen. Also, I think the character has too many live points. I didn't die once.
And maybe the player can hit the tree when he has some size. I sit near the tree and I'm hitting the tree all the time gathering bullets from easy enemies because I was small.