Confused

Okay guys new to this so if this is ridiculous let me know lol I’ve always watched my 600lb life. He puts them on a strict 1200 calorie diet to loose weight. If I’m considered “ overweight “ which I’m not but my dr said I am I’m only 148 5’2 then why is 1200 unhealthy for someone like me ? Is it because they are bigger or something to do with that? Idk I’m all confused in all of this 😭😂