There's over a million photos of Beyonce you google the two that shows what you are trying to prove. You are using photos from her skinniest period from a certain angle. Other skinny broads have mute asses, there's some meat on that. To add to the pregnancy thing, pregnancy can leave you with wider hips will make your ass bigger. As can any type of weight gain, that's why women who were skinny can grown fat asses as they gain weight, with some training they can turn that into a phat ass. Specially if they have the best personal trainers that money can buy.
"Hey let's find photos of Beyonce at her skinniest from a certain angle in semi-thick jeans and compare it to her at her thickest after 3 kids, 14 years and significant weight gain from a certain angle in leggings".
This is also Beyonce in the same period