this is pretty cool, i actually built a similar game in flash a while back, the problem with the version i made (which you address here) is that it always ended up with my dot and one big ass dot. hence allowing me to win or lose very easily. with your version you not only spawn circles based on your size, but you also include shooting. nice work!

i think if you modify the shooting mechanic to work with the mouse, it would make the game a bit easier to control.