My response may not be a popular one.
My mother was kind of like that, but really was sadistic in other ways. So, I speak from experience.
I'd call her on it. Just flat out tell her that her comments are rude and uncalled for. If she's overweight, I'd tell her that if she's jealous, she needs to find a constructive way to deal with her feelings of self hatred, and not put you down to make herself feel better. And if she doesn't stop, which, I'm sorry she likely won't. I'd stop visiting her. Harsh I know, but here's the deal...
Your 21. Its very likely that if your mom talks to you like crap now, that this didn't just come out of the clear blue sky. She's probably always talked to you that way. I'd imagine that all you've ever known is a mother that says hurtful things that moms shouldnt say to their daughters. Am I right? So in a way, you are used to or accustom to these remarks that damage your self esteem. Please, let me know if this is the first time your mother has ever been hurtful because I would find that shocking that a woman with a history of supportive words suddenly became mean.
Also, assuming your mother isn't severly socially inept, she's must know that her comments are rude. Seriously, its not like those comments could have been meant to be kind. She was trying to be an @$$. Even if you call her on it, she will probably just get mad that her verbal punching bag of 2 decades wants the verbal abuse to stop, OR if your mother is more of a cryer or plays the victim, she might cry that
you've hurt
her feelings by pointing it out, therefore making you feel bad for calling her out on her behavior.
Chances are she won't change if this is the kind of person she is. That's not set in stone, though. I'm telling you, she knows she's being hurtful. But maybe she does feel guilty about being a mean person to you, and wants to change, but never really knew how to go about it. Maybe confronting her will help, but be prepared that it won't. Sorry

Assuming she does not make an effort to stop talking to you this way, you should ask yourself, would you keep a friend that talked to you this way? Or would you allow a coworker to talk to you this way? I certainly wouldn't. I either had to accept that my mother was going to spend the rest of her life treating me worse than I would ever let someone else treat me (she refused to go to family counciling, but maybe that's another option for you and your mom) or I would have to tell her I would no longer allow her in my life until she was willing to take steps to treat me nicer. She made it clear that as my mother she could treat me anyway she wanted, and so she is no longer in my life.
Its been about 5 years, and my self esteem is slowly improving. Also it feels good to be free from all the fights (I would go back at her when she made rude comments) , drama, and stewing over things she said. My life is much more peaceful now.
I hope your situation works out better than mine.