It was kinda lost on me until now the meatball thing, but it really does illustrate just how poor Kano (and as we now find out, Usui) are. Ishii's meatballs are these tiny pre-sauced meatballs they sell frozen in Japanese supermarkets, and are commonly found in bentos.
Kano and Usui saying they can eat an entire bowl of rice with just one is extremely frugal, a bento box would normally contain several. And to top it all off, they'd eat another bowl with just the sauce? It should really hit home just how much of a struggle it was just to put food on the table.