This is definitely fun! I want to keep playing, but I just got to the first checkpoint and I've got to leave soon. So I've just got a couple of questions / comments:
At first, it wasn't clear to me that I could shrink if I stopped shooting. Once I figured this out, it became a much more natural experience, and I was pretty absorbed by it. Maybe put in the title screen 'Hold H to evolve, release to devolve'? I was ready to give up much earlier before I figured out this distinction.
I do have a qualm with the evolution mechanic itself. It works great - you're rewarded for being awesome, and it gives you a panic button type survival mechanism. My problem with it is that I don't understand WHY I grow when I shoot, and shrink when I don't. I like the word 'evolve' here, but I fail to see how it applies to a piece of technology like your ship. Maybe if the player controls some kind of
Mutalisk, for example, the evolving thing becomes much less abstract.
All in all, very fun. You have something here!