I've noticed an interesting social phenomenon where mothers tend to discourage their own sons from working towards achieving more muscular physiques. I have observed it with my own mother who tells me things like: "why do you work out? girls don't like big muscles". My friends who work out have also experienced it to various degrees. Additionally, I have seen it mentioned online plenty of times, to the point where it is a running joke/meme in certain bodybuilding forums.

It doesn't make sense to me, because I would think that mothers would want to support their sons in being as sexually successful as possible. Additionally, this mindset isn't consistent with other desirable physical attributes. For example, mothers seem so to be proud of their sons when they are good-looking and/or tall.

Does anyone have an explanation for this?