Max size? Does her capacity improve? I did an inflation at the start, water from the sink in each way (need a 'cancel activity' button as I restarted her inflation SO many times trying to stop when she was full), but once she got filled, I didn't see much change.
Ayume starts with a very small max capacity, if you keep her belly full throughout the day (and fast forward time by resting) she will be able to get much bigger (up to 20 times bigger)
