Is it because men are evolutionarily not attracted to a woman that looks pregnant because they cannot be impregnated at that time?

Is it because they lack a specific look curve wise? If so, why are skinny girls with barely any curves still seen as more attractive?

Also, African American men are more fond of overweight women, is it more of an issue of social hierarchy or conditioning?

Or is attraction a function of value instead of the reverse, and somehow excess weight lowers value?

More scientific answers please rather than stereotypes. The concern is more of WHY than WHAT