Wow, that's a great idea for the theme, and that level of polish is through the roof, great job!
Sadly it's a bit too tough for me, I couldn't even beat the first boss. I'm good enough to not die but not good enough to grow sufficiently, so I can't comment on much more than the graphics, vibe and theme implementation.
I feel that the length of the fights can suffer if the player gets stuck in that purgatory state (not big enough but not dying either), but I like the idea that the closer you are to defeating the boss, the riskier it becomes because it's harder to avoid the bullets. That's a solid mechanic as long as you can keep the length of the fight in check.