Another Godot user, nice!  Really nice game: polished, sprites are cute, great concept and base mechanics, as someone already said controls can feel a bit slippery/floaty but I got used to them pretty quick, I really like the color palette and the geometric/minimalistic background that fits very well with the music, the parallax background and it runs smooth!

I playtested for a little bit (to find something that could be useful for you) this is what I noticed (maybe you already know about them):

1.  you are on a moving platform and you are not moving concerning the moving platform, you shrunk -> (probably because of the different inertia you gave to big and small states) you are thrown forward (in the direction of the platform movement), I would expect that if I'm I got zero momentum respect something shrinking does not change that
2. I was able to do the following (I think it could break some levels, for example, I used it in level 2) a combination of jump and un-shrunk in constrained space: 

Anyways, super great job, really enjoyed it! 

Liked the shrinking mechanic. Nice art and music. Great job! 

Wow, really good gameplay features like shrinking mechanic! Good job!

Great game! Love the shrinking mechanic! 

Cool game concept! Difficulty curve cld be improved tho tbh

This is such a great game concept.  The character is so cute and I love the sound too.  I had trouble on the 2nd level and wasn't able to make past the spikes after getting the key.  I think I need more practice :-) Great work!

I love it when you can do things with the character that bends the natural laws of physics! The general physics were nice and the theme was unique and elegant in my opinion. Well done!

It felt a bit stiff, but the art and the character were really cute!

Cool game! Nice level design, good audio. The only thing that I would change would be to change sizes with one button.

I looooove the concept for this!! Also the character bobbing when you jump was a really nice touch!

Really good game! As everybody noticed, the controls were a bit off, but for me it wasn't a game-changing issue, great work with the graphics and the sound, keep up the good work!

Fun game and concept, but jump feels way too floaty, and also it is hard to tell between the actual platforms to the background.

But overall I had fun with this one, good job!

I love this concept, my only complaint is the controls. As others have already mentioned they feel "floaty", and (if you are planning on tweaking the game after rating has ended) I might suggest that grow/shrink is bound to a single key press as a toggle. Otherwise love what you have done here!

Game controls freeze frequently but otherwise good.

Man your right that little dude is floooaty and sometimes frustrating. Thankfully the levels were a really good length so it was doable and felt really rewarding to get to the end. 

Not sure if it is just me but the tutorial never ended so I was confused on how to 'use' the key for a bit.