Why does my mom criticize fat people?

She works out at this public gym and sometimes I go with her. She has lost a lot of weight and she used to be pretty fat. Now she keeps talking about how skinny she is and how people compliment her. She talks about this everyday, but she says she doesn't like when people brag about stuff. She also talks about how there's mostly 'fat' and 'disgusting' people where she goes and always makes these faces when she mentions them. Why is she like this?