A few questions about the sin Gluttony?

I read vaguely over it and got a gist that basically if you are living for food and anticipating when you eat all the time than it is considered a sin, and also if you over indulge it is a sin. Why is this called one of the 7 deadly sins? Why is this sin worse than others? Also does this mean if you are overweight than you are of sin? I never really knew this was a sin and now that I realize it is I'm wanting to change my ways for good to live by Gods word but can't understand it well.