That was very fun! The levels were challenging, but fair, and the mechanic made for quite interesting designs.
While most of the physics made sense, one thing I had trouble realizing was that, during long horizontal leaps, holding in the direction of the leap can actually slow down the character.
The vibes are immaculate! The art is extremely pleasant, and the Kevin MacLeod is a great fit. I especially like the art on the serpent.
While most of the audio balancing is fine, the death sound and the level start "click" sound are a smidgen too loud; they regularly peak the audio above +0.0 dB.
I'm not sure if having big grow and big shrink be binded to Shift + Q/E makes a lot of sense, given how much they need to be used. I spent long amounts of time with my pinky on the shift key, and it wasn't the most comfortable thing.
All-in-all, fantastic! Great work!