Now, this picture right here is something to enjoy. The linework and shading are phenomenal, and the color two steps past that.
Personally, though, I didn't imagine an anthropomorphized Pinkie to have such a grand bust. Perhaps your logic derives it from the unholy volume of food she consumes? Or, as is possibility with anything that has big bewbs, was it "just 'cause?"
A strange conundrum to me, but lovely to look at, nonetheless.
A large bust is fairly common for humanized Pinkie, just as a flat chest is common for humanized RD. I personally believe it's from the idea that she'd be physically attracting, which would help her to make friends easily. I dunno, it's just my thoughts.
Indeed, not being hard to look at makes it easier for one to receive a return on a started conversation. However, just by how much she's always moving, in combination with her assumably ridiculous metabolism, I personally think it'd be closer to an average size.
Please forgive me, I don't wish to sound argumentative. I don't often get into a philosophical conversation about pony boobies.