Eat pickled onions, that's the most obvious one. Alternatively at some point Jun will try to perform a well known anime meme, as a result of which you will get the chance to buy an old stove from Needled!ck (aka the Landlord), for like 400 or 600 dolars (I forgor the exact price). That will unlock the rythm based cooking minigame, in which you can combine food items from the grocery store and cook meals that will likely max out your hunger meter, and drasticaly increase the capacity of your balls.
Hope this helps.