1） plump,bony,skinny　　plump,丰满的.she is plump.

fat: 含有歧视的意义

he is bony. 含有贬义

bony skinny 均含有贬义

而thin勉强可以接受

slim 是一个褒义词

2）两个句子表达减肥

I am going on a diet.

go on a diet:节食

diet:保健食谱,特种饮食

I am supposed to be slimming.我需要减肥.

-Do you want some more?

-No, i am supposed to be slimming.