Here's my feedback:
- Adding an effect to changing size might be a good idea , making it randomly timed is kind of awful though
- You can add like flashing lights to character to warn the player that she's gonna shrink / expand
- sometimes the double jump I don't know why ( even though i tried with full size and small size)
- There is a tile near second key that has no collisions and when i stand on it i fall and die ( not the one that leads to chest)
- there is a key that's in the middle of the tiles I'm not sure how to get it
- The coin sound effect becomes very annoying after taking a couple of coins , maybe add a different effect or increase pitch according to how much coins you took
- The pixel size is inconsistent and you should have drew a smaller sprite with same pixel size instead of just decreasing scale by 2
- The losing text is kinda weird that it's made by ai , it's not crisp at all and too large
- The back button keeps changing position with the player which is kind of annoying
- The background is kind of blurry and doesn't loop around which is what parallax background are supposed to be
- since double jump lags sometimes you can do a triple jump
- The first tile with the key has tilemaps with collisions that aren't correctly set and are very annoying to deal with
Good job overall on your game.