Why do we skinny shame now!!

I saw a picture on instagram of a heavy set girl and she was going on and on about how she finally came to love her body. Everyone was in the comments happy and telling her how beautiful she was and all that. Then another more toned girl did the same thing saying how hard she worked for her body and everyone called her a slut and telling her to put some clothes on. Why are we only body positive for certain people now....?