Hi Ireie, what a difficult position to be in! I feel for you. I'm no expert but here are my thoughts.
First, it sounds like you are saying that it does not help your daughter to tell herself to "let it roll off her back" or "not let it bother her." Do you think it would help to have some discussion about why these kids may be picking on her? I have a feeling that your daughter would say that it is because the kids don't like her; however, this could be your opportunity to help her think of alternate reasons they might also be doing what they do (that have nothing to do with your daughter). For example, "the kids are doing it because they have low self-esteem themselves," "the kids are jealous because _____," "the kids might be having their own problems (at home, at school) and are taking them out on me." These suggestions won't fix the problem but may help your daughter to think about it in a different way so that she doesn't take it so personally (and it doesn't negatively affect her self-esteem) - because we know this issue really isn't about her.
Second, it may also help to brainstorm all of your daughter's strengths so that when these kids start picking on her she can remind herself that there is more to her than her "weight." I'm sure you already do this but sometimes it can help kids to make it very explicit. For example, write the list out, make it fun colors, draw on it as well. Then when she is feeling bad at school she can pull this paper out of her backpack to remind her of all of her great qualities.
I wish you luck with this situation and hope that it gets better soon! (((Ireie and daughter)))