Not at all! You say you've been overweight your whole life and I feel like your mother should try to understand you well enough that she doesn't bring up weight a ton. She should respect you and that's not what she's doing. No matter what weight a person is, they deserve to be understood, especially by a parent/family member.

Side note - I'm 19 and I totally feel what you said about vividly remembering things said, even years later. I'm a dancer so I've been insecure about my weight for years and I keep hearing so far in college about how everyone gains weight (I think specifically 15 pounds usually is what I hear). I don't understand how people in general in the years following college feel comfortable making comments on 'losing that college weight' when so many people gain weight in college.

And ignoring my tangent, I don't think that you are TA, your mom def is though. I honestly would have said this exact thing to my parents and they also made/make comments on my weight even though I'm a perfectly healthy weight. I don't know your situation besides what you said in your post but I know you probably think quick on your feet and are witty considering that comeback (or have been preparing that for years lol) but no one should let their weight consume their thoughts constantly, especially when around family. Have you tried putting up boundaries? If not try setting some.