Even a C- or D- cup would already be smaller without being actually small and, i'm with enamel on this one, far better looking. I'm similarly not a fan of silly large features, although i get this is at least to some degree Honey Select thing.
Counterpoint: one is a ditzy blonde and the other is a moody goth girl. Out of them too, Mina's type is the one who more typically gets saddled with large breasts.
That said, their visual similarity is kind of a thing, so any changes should likely affect both. And getting bit smaller wouldn't hurt them (the opposite if anything, it'd be a literal relief for their backs