I loved how you can use the environment to progress! Pressing the boxes to grow/shrink is one thing, but then also using that mechanic to launch yourself or other pushable boxes was a smart move! It does lack some visual elements though.
Great level-designing and the levels are short, and the game is relatively short -perfect for a game jam.
There's not much to say about the existing mechanics, they are well made and polished, and it fits the theme. (only lacking the "built"-part) ->idea for that would be, you need to place/build the boxes you can grow and shrink, though that would lead to more difficult puzzles.
Summarize: Good and polished mechanics, a bit simplistic but effective use of existing mechanics. A typical side-scroller puzzler (lacks creativity) and there's very few interesting visuals.
You should be proud though! There's only so-so many things you can accomplish alone with this time-limit, and you have certainly accomplished alot! congrats :)