What’s a snappy comeback when someone says ‘Have you gained weight?’?
”Why, have you?”* Said in a genuinely enquiring voice.
The key is to make it sound like are really concerned about them, rather than using it as a comeback.
They will be thrown off, and hopefully also realise what a rude and intrusive question it is.
PS. Yeah, I’ve given up with telling people I don’t believe in snappy comebacks. You’ve worn me down.
*Obvious disclaimer to simply answer truthfully when asked by a medical professional. No-one is paid enough for that.