Honestly, I always thought Mugi was an 'S'... until she was begging Ritsu to hit her, which clearly makes her an 'M'. Who knows, maybe she's a bit of both.
Mugyuuuuuu"Tsu... Tsumugi-sama... Y... Your grip really is strong, isn't it? ... Gu...!""You think so? ♪ I'm surprisingly strong, huh? I can even crush an apple!""Eh!? Y... You're kidding, right...?""Fufuu, I wonder? But when you have two eggs this soft, it's simple, just like that ... *crunch-crunch* ... see? ♡""Ah! Wai...!"*crunch-crunch*