Whatever you say you'll be wrong. If your answer is "You are not!" then she will blame you "You are not looking at me at all. Am i not pretty for you?" If your answer is "Yes you are!" then you are screwed. If your answer will be silence then she will do the same as the answer would be "Yes you are!" but she will add... you are a coward.

Would it be weird to ask if she wants to workout with you if she feels so insecure?

You are not fat, you are bulking, keep doing that

(Ronnie Coleman's voice) LIGHTWEIGHT BABY!!

Just tell her it's going to the right places. My wife gained weight during pregnancy but she's hot as hell still, that's what I tell her.

Remind her of how big her hugs are now.

Are you solutions-oriented about it or in the feelings stage?

Dude if your GF is looking for validation and she’s not fat, your job is easy as fuck here

Making beeping noises when she backs up

It's a trap. There is no good answer. You need to turn around and walk until you pass out. It's over.

"It's been a weird few months. I'm going to work on eating better if you want to join me."

Start singing and dancing to “Fat” by Weird Al. Bonus points if you have a few friends with you to recreate the moves in the music video.

You ain't, but join the gym to keep it like that.