Leaving aside the unpleasant sizeism of your mother’s friend’s comment, you embrace the frankly unlikely types of suspension of disbelief you are required to do with opera, or you don’t.
Yes, a forty-something, matronly soprano from Kansas is sometimes playing a wispy consumptive Parisian seamstress or a 15 year old Japanese bride of convenience, or a woman is playing roles traditionally written for castrated male singers, or we get heavily involved in a frankly mad plots in which a vengeful gypsy, charged by her mother to avenge herself on the noble family who burned her at the stake, accidentally throws the wrong baby into the fire, or two men test their girlfriends’ fidelity by pretending to go off to war, only to return dressed as sexy Albanians who threaten to poison themselves if they don’t get a kiss etc etc.
It’s mad. But I think easier to suspend disbelief when you’re at a live production with singers and chorus and orchestra and costumes and sets and a whole audience listening, and when you know the music there’s the pleasure of recognising different tempi or interpretations etc.
I mean, I can look at that clip from the 1974 Norma and recognise that it’s in one sense an overweight woman with very 70s make up and a terrible headdress singing a hymn to the moon among a lot of chorus members in Druid costumes that look like sheets flapping on the clothesline, but also that it’s an astonishing performance, full of heart and skill and guts and intelligence, and that it takes serious nerve to sing that pianissimo outdoors in a windstorm! And is very beautiful.