Tsukushi's Lunch Box

Mr. Shibata Akimasa, a circular lunch box craftsman, decide to make one specially for his daughter Tsukushi. However, there was a bit of a problem. Children develop quickly, so of course over time the amount of food they consume increases. Typically that's a happy process to watch unfold, but a bit of a problem when making lunch boxes. The right size will be the wrong size in a year, and a large lunch box looks odd in the hands of a six year old. Of course we want the lunch box used for a long time...