It's all about personal preference. A person you find perfect may be extremely ugly in the eyes of another, and vice versa.
Personal preference is partially formed by what you see around you, as you tend to lean towards what is possible. That's why more and more Americans see 'heavier' partners as attractive.
Personally I do not like people who are under- or overweight. I don't mind lean girls that much, as people tend to gain some weight when they age still. As long as it does not become really skinny. But I don't like girls with 'round' faces much, as that tends to mean they have a tendency to gain weight later on and become overweight.