> . In some cultures, thin is attractive. In others, fat.
Specifically, in cultures in which the majority of people are overweight, where obesity-related causes such as heart disease are the most common causes of death, thinner than average tends to be a) healthier and b) generally considered attractive. In cultures in which food is scarce and malnutrition is a widespread problem, being heavier than the average starving peon is a) healthier and b) generally considered attractive.
Some people enjoy an excess of whatever is physically good, as if their libido thinks "if some is good, more is better". A plentiful supply of milk is good for reproductive odds, some guys enjoy very large breasts (where medium is sufficient); if a lot of people are malnourished thicker is healthier, and to some people very thick is very attractive.
Fashions vary, but standards of what turns people on is more the same across cultures than is different. (Especially when you factor in that "healthier than average weight" is the criteria - that may be thicker or thinner, depending on if the average is obese or malnourished).