It seems the word "mansion" has been borrowed in Japanese as a term for a "large apartment". I don't know if it's a tongue-in-cheek humor thing or a relative comparison in a crowded high-rise landscape.

I think the joke here is that Koyomi and Madoka are way less affluent than Ritsuka and Emma (who both go to a prestigious and exclusive school).

HawkeyeNFO said:

It seems the word "mansion" has been borrowed in Japanese as a term for a "large apartment". I don't know if it's a tongue-in-cheek humor thing or a relative comparison in a crowded high-rise landscape.

I think the joke here is that Koyomi and Madoka are way less affluent than Ritsuka and Emma (who both go to a prestigious and exclusive school).

It's not even a large apartment, just an apartment building. The meaning changed when the word was borrowed. It's a pretty common word.