Clean and minimalist, I love it! The base game felt a little basic, but on the last levels I eventually get into the speedrun aspect of the game and took pleasure in optimizing my level-design like a Trackmania course. The game has a great physics for that!
I didn't use that much the shrink mechanic, only occasionally. The best strategy ended up building momentum thanks to gravity or boost, and keeping it. Also the idea of having to build with random pieces is interesting, but in the end not restrictive enough: it's really easy to put aside something we don't want to use, and sometime I would just draw until I get something useful.
I must also mention that the UX is kinda disconcerting. I was very lost at first on how to play (even after seeing the tutorial pictures on the game page). And the first time I finished a level, I had troubles exiting it to select the next one.
Overall it's a simple concept, but well executed in a neat small game. Well done!