I liked the chill background noises. The mechanic was a great idea, with different abilities and difference in sizes also creates for some good idea for puzzles in the future. The jump could be finetuned a bit to feel more adaptive, but it was a nice platformer anyway! :)
Once small bug I noticed was that once you die and press restart, the diamonds don't reset back to 0, so in theory you could break the game by collecting ton of diamonds, dying and redoing it again and again, but it would obviously take ages :D
Main menu was a bit hard to understand, but otherwise nice looking game. Brought me a bit back on the old sonic background :)