Nice game! The concept of changing the size of your character to progress through levels is interesting and works well with the theme. The artwork and audio is also very nice.
As someone else said, the controls feel slightly unresponsive, like the character feels too slippy. My guess is that you used physics to control the characters movement, and I would suggest translating the character directly instead to get tighter-feeling controls.
It's very cool how the first three levels introduce the player to differently-sized characters, and then on the fourth level you can't progress until you figure out that you can use the flower to change your size. I love stuff like that that organically teaches the player how to play without directly telling them. My only suggestion here would be to make the flower visually stick out more, because it blends in with the mushrooms and stuff also in the foreground.
So overall, nice job!