I'm not a big fan of the theme here.
You have a bat. You give the bat Air and it becomes bigger. Huh? Are we talking about a bat or a balloon?
Other growth creatures make sense like Forest Spirit getting to grow.
This is
exactly what I was going to say.
You could get so much more creative with this card. What about gaining
x|0 for a certain amount of darkness quanta, as bats gain an attack advantage in darkness? What about a swarming function similar to scarabs (minus the devour ability, naturally), where the damage dealt is x + 2, where x is the number of bats in play?
I think the idea has a lot of potential, but it needs to be reworked a bit.