I'm coming across the same problem. Even when filling all 3 areas completely, I can't get her to drink any more green, and therefore no more water.
I'm gonna try doing it so her belly is the last to grow to max, and see if that changes things.
EDIT: Still nope. This is what I've been getting anytime I try to use the green potion at the listed size. And she still won't drink from the spout. Hopefully the rest of the text confirms she's at maximum size.