I don't quite get the short focus on how much Merry and Renko eat at cafes. The way Ayaya looks at it, it seems almost sinister. It's just poking fun at their appetites, but is there something else I'm missing?
If I had to guess it's probably a joke about them eating so much but not getting fat and Aya getting mad about it. It may also be a more subtle joke about the relation of Renko and Merry to Reimu and Yukari. There's a fandom joke about Reimu eating a lot and not getting fat.