
People like this are the reason why vanity sizing exists
Size labels on clothing shouldn't affect your self esteem to that extent. If you're a small, you're a small, If you're a large, you're a large and that's it. Size labels are there to see which size you fit into, nothing more nothing less. I know she's just a child, but the mom should teach her that. Because once she grows up and starts shopping in the adult section, she should be prepared for it.
I know this probably sounds unrelated to vanity sizing at first, but I think it's actually the same underlying issue. When we teach girls that being a “small” is desirable and being a “large” is something upsetting, we're giving those labels an emotional value they weren't supposed to have. Then, as people get older, they understandably start wanting the smaller number/letter because it makes them feel better about themselves. That's exactly the kind of mentality that creates demand for vanity sizing.
We should be teaching kids that the label doesn't matter in the first place.
Teach her that large isn't an insult. Teach her that small isn't an achievement. It's just a shirt.