It correlates with socioeconomic status. During the middle ages, fat women were considered attractive because it indicated that you had ample food available, a sign of wealth. Now thin women are desirable because it shows you have time to exercise, money for a gym membership, and don't eat at McDonalds.
Same goes for many other physical attributes. In much of Africa and Asia, whiter skin is attractive because white foreigners have all the wealth and power. In the U.S, tanned skin is attractive because it shows that you spend all day on the beach, which means you're wealthy enough to live near the ocean and leisured enough to spend time in the sun.
Humans seem biologically programmed to favor mates that are wealthy and powerful. However, the specifics of "wealthy and powerful" aren't hardwired in, so we take our cues from physical characteristics that appear correlated with wealth and power. When I was a kid, the girls would make fun of all the geeks. Now that geeks tend to be millionaires, they've reversed their tune.
