Loved the art and music. I dove straight into the game so didn't see the direction instructions on the main page so spent a while trying to figure out how to jump (P is a bit of a odd key choice). Given you had in world instructions for the making a bone bridge, adding one early on for the jump would have helped a lot there.
The idea of using up bones to make paths and the different sizes/abilities you could have at each level of bones is interesting in concept. I feel like this could lead to some interesting puzzles in future levels.
The only remaining comments are on the sound effect (bones rattling) being a little loud and can be annoying if you are stuck in that mode too long, and there is no indicator on how many bones are needed for the next level change.
Great job overall and again love the background music and pixel art style.