There's a core of something interesting here. but it could definitely use some work:

• There's never any real reason to use the smallest/largest sizes. Having a smaller amount, like 3 or 4 would make it easier to design the game around it and give each size a unique gimmick, like the two largest sizes being able to push boxes, but the largest one breaking platforms.
• The blue areas seem very arbitrary and feel like an afterthought. They don't feel like a natural way to restrict what your size is unlike the sections with small platforms. They could work, but as more of a puzzle element, like setting something up as a size 3 block to get a size 4 block over the blue area.
• I like the "increasing weight by falling" mechanic. It'd be cool to see it expanded away from just buttons into other stuff, like platforms/boxes/etc. That should help make it easier to design levels around the weight mechanics. 
• There is a bug where the blue areas can kill you if you fall far enough due to the weight mechanic. I'd suggest making falling weight a different variable that the blue areas don't check to avoid it.
• Having a long section of platforming where it's easy to die, and you get sent to the start of the level isn't an interesting form of difficulty, it just makes it tedious and annoying. You want to have as short a time as possible between an area the player failed at and their spawn point so it's faster for them to learn the difficult section and so they don't get bored.

I hope this didn't come off as too harsh, this is a pretty good game for a first time jam entry. I hope you do more jams in the future, they're a great way to get a lot of skill in making games!

Really liked the first bit of playing, the scaling and different mechanics that came with each size was fun but the platforming was pretty brutal and I'm not a very patient player, but otherwise really loved the mechanics and premise! Sound design was pretty good too!

Nice entry, I like the inherent trade-off between the sizes. It's a good base for a game.

I disagree with the suggestion here that only a few are enough - you can make a great game with many sizes as long as most of them have a reason to exist... so I don't think it's a bad design, just gotta play with it a bit to make most of them viable!