There are various ways of say them: to be skinny やせている (the most common)、細い、スリムな、etc *引き締まっている/引き締まった is used for "to be slim and muscular". to be fat 太っている (the most common)、ポッチャリしている (sounds a bit cute)、恰幅が良い (mainly for men)、貫禄がある、etc *We don't say ふくぶくしい, at least I don't. 体格が良い is also used for "to be muscular". **There also are some insults that I didn't write. I hope this helps you.