Congratulations to dd on the new baby and the weight loss!
So sorry about mil's behavior/obsession with weight. Since she struggles with weight, herself, over the years, she may really think she's being helpful/ that her experience can help. Maybe she thinks someone should have pushed her to lose weight earlier in life. Who knows?
But dh has let her know her comments are hurtful and still she continues. So, imo, she's not totally innocent here. Perhaps she's taking out on dd what some people said to her when she was younger and heavier. Again, hard to know.
What I'm wondering, though, is why does dd continue to visit her once a week and talk to her on the phone, as well? Why subject herself to this rude behavior? I agree with those who say dd needs to take a break from her gm for a while. Or if she does talk to her, keep the conversation short, and end it as soon as mil brings up the topic of weight.
But as pps (previous posters) have said, dd is an adult, so only she can decide what to do.