I enjoyed this one a lot quite a bit.
Very interesting retro art style. Alien mothership thing looks very cool, and powerups look very distinct. SFX are fine, and contrary to Nitro-Gen's review, I thought the music fit with and indeed added to the game.
The fact that the player orbits around the Enemy ship (as opposed to the Earth or just flying around) was a cool idea. Made for some interesting moments where I had to adjust my orbit and the direction I was moving to hit enemies.
I'd say you just about managed to include the theme with the ship getting bigger when you are hit, although it does feel tacked on.
For improvements, I'd say some more variety in powerups, such as changing the projectiles you shoot, would be welcome. I think I only found three powerup types in my playthroughs.
The UI, while pretty, can be hard to read. Does it say what powerups I have anywhere?
It would be nice if I did not have to press [K] everytime to shoot, maybe let the player hold down [K] to fire when available (the infinite ammo powerup helped a lot, but still).
Finally, I notice that the name you give your Pilot is not saved between playthroughs. I did not realise this and ended up not having my highscore where I beat the game registered :(
With all that said, you've made a fun and original submission. Nice job!