Not wrong, really. Just know that there's no way you're going to get a positive reaction out of it. It's more an etiquette thing, but honestly if you feel that your girlfriend is gaining too much weight and it will negatively affect your relationship, you should be honest about it, and if she reacts in a bad way then you may as well cut ties with that person cause they're never going to be the person you want them to be.
Which is hilarious because I always hear the reverse. Women talking about how their BFs are getting fat, and even saying it to their faces.
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TheRock ~ Slow dramatic zoom-pan. Doesn't phase the hooded man.
"You have issues." - MWC. Pot. Kettle.