Zingers won't help really. All it will do is make them nod knowingly and say "ah, she's under stress" or "it must be the diet that's making her b*tchy".
And really, zingers just lower you to their level.
I'd keep it short and honest.
"Mom, you're not helping me or being supportive like this. My weight/diet is not open for discussion any more. Let's change the subject."
"Mom, I called you because I was happy about this opportunity. You're not being supportive, so I'm going to go now. I'll talk to you later."
"Mom you tell me I'm fat and then you push food at me - it's very hurtful that you don't support me. Please don't do it again."
And there's a huge amount of value in remaining calm, and hanging up or walking away. Once family members realize that they cannot bait you into reacting (because your reaction is part of what they *need* out of the interaction), they're less likely to keep doing it.
I'm sorry that your mom is less than supportive ... and it's no fun to have to be the "grown up" with your own parents.
Good luck to you in standing up and letting her know that her behavior is not acceptable.
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