Agreed; I had thought of something similar as well. Make both max size and production increase based on how you milk the barista.
More than 95% full = slow max size increase, until milked down under 75%
Milked from at least 80% full down to less than 10% (drain the tanks to near empty) = small production increase
For added difficulty and strategy, add an "overstretched" debuff bar - increases as her max size does; decreases when not near max size. High debuff causes more happiness (smiles%) lost. ..... Would prevent you from trying to race to max size.... might make it too difficult though, esp late-game hard milky or chaos milky modes where she fills up almost instantly XD