Definitely a good start. Would be nice if the Milk machine actually listed how much was ready to be drained (always at 0 regardless of how much you're holding).
Another thing that'd be good to consider is something indicating how much milk you'll produce before the pregnancy ends and you'll need to reset. Possibly with "max size" in consideration as well.
I would LOVE to see this fleshed out more. It's got a solid foundation, and definitely warrants being built into a more complete game, even if it's just "find more monsters, breed more".
If you get to a more full fledged development, here are my suggestions:
- Current/Expected/Max milk indicators
- Milking machine art changes depending on how full the character is
- releasing a grown monster allows bigger variants to come out (with appropriate bonuses for breeding)
- capturing a monster allows you to turn it into food for other monsters, leading to carnivore/herbivore/omnivore variants.
- more art of the protagonist at much bigger sizes (okay, that one's probably a given)
EDIT: Also, I think the breast-expansion tag is warranted, which will help give your game more exposure.