From BBC News:
A study of 61 male university students found those who were hungry were attracted to heavier women than those who were satiated. The hungry men also paid much less attention to a woman’s body shape and regarded less curvy figures as more attractive. Although it is not clear exactly how hunger exerts an influence on attraction, past research suggests social, cultural and psychological factors are involved.
In some societies where food is a limited resource, such as the South Pacific, higher body weights are revered. In others where food is abundant, such as the West, lower female body weights are preferred. Evolutionary psychologists believe this is a survival preference. What you are looking for in a mate is the best chance of healthy offspring and in an environment where food is scarce, a heavier woman is deemed a safer bet for this.