Augh. So sorry that the family has put this on you. My mom didn't even have the flimsy excuse of being Italian (not that that should matter). My entire youth it was, "you'd be so pretty - if only your face was thinner". I would eventually learn to torque her out by correcting her incorrect lack of use of the subjunctive mood by saying, "I think you mean, "if only your face WERE thinner, don't you, Mom?"...but I digress. When I became fit, I got the about face, too...picture sitting with the family, watching a female Olympic diver and having her grouse, "she's so muscular, she looks like a man", with a pointed glance in my direction. Well, she didn't look like a man, but she looked like ME.

Ignore it. Smile sweetly and say, in the blandest, cheerful-est, vaguely distracted tone you can muster, "Interesting you should say that. Thanks for the input". This gives them nothing to argue, but also gives them the (correct) impression that you don't really place much value on their opinion.

If you get really cornered, you could let them know that their opinion is very different from that of your (doctor, nutritionist, personal trainer, running partner, pool boy) as appropriate and/or inappropriate.

If you get more cornered than that, call a taxi and get the hell out of Dodge.

One of the big lessons I learned in interacting with my Mom was that she has incredibly low self-esteem, herself. And that any question that she asked that started with the phrase, "Wouldn't you RATHER....??" was best answered with a gentle, "no".

Pay back your family's unkindness with kindness by not inflicting low self-esteem on the next generation. They may never change but that's out of your control.