This is a fun, simple game! Perfect scope for a jam. I like the sound effects!
The one piece of feedback I think I have is it's unclear why you want to use the shrinking bullet. It seemed to me like objects died in about the same number of bullets no matter which ones you hit them with, so I'm not sure why you'd pick a shrinking bullet over a normal bullet. Maybe something like asteroids take 10 normal bullets to hit, but if you shrink them one level they take 5, or if they shrink two levels they take 2 or something similar.
Awesome job!