I finished the demo. I must admit the Big Alice was rather powerful against the last boss, in particular the fact that she can't be knocked away was a major advantage. I earlier suggested either charging the bar faster or make the transformation cost less. I'm definitely suggesting the first option now, if the big transformation lasts longer you can cheese bosses way to easily.
Actually, the Big Alice seems powerful enough against bosses, but not so powerful against cannon fodder enemies. Sure, she kills them faster just like she kills bosses faster, but that never seemed so important against cannon fodder enemies.
I've yet to use Small Alice except when needed. One problem is that you can't use her superior evasive abilities to actually solve problems. Big Alice can kill enemies faster and thus be used to quickly solve problems. Dodging will however not solve the problem, the enemies will keep attacking until they're dead. Unless Small Alice can kill enemies before reverting, she will not help much. Maybe she needs a longer duration than Big Alice?
I somehow managed to forget about the projectile attack. It's near useless against the first enemy type and after a few rooms I failed to remember it. I'll try to remember about the projectile attack next time I play.
I like the actual ideas though. If you can get the balance right the game will be really fun.
Edit: I replayed it and killed the last boss with one combo as Big Alice. That definitely didn't happen before.