I love the art and the sound when the enemies die! Also the controls feel snappy. I do have some suggestions though:
I imagine you thought spamming was going to be bad, but I think if you made the weapons automatic it would be so much easier on the fingers. You can always balance spammy shooters with recharging ammo or something.
Also, maybe at least one more type of enemy that challenges you when you're bigger could be interesting.
Good foundation though!