My breast are softer than my upper arms so that's just a myth
I've learned that in Japanese works (probably Asian in general but my exposure to mainland popular culture has been much less) "they say" is basically shorthand for "arbitrary bullshit being taken as gospel for no obvious reason."
Gets downright weird in more dramatic stories where characters routinely base plans with much at stake - anything from someone's life to the fate of the world - on such usually totally unsourced hearsay and superstition; though being rather cynical I'm inclined to suspect lazy and uninspired writing is often at play in those cases...