You know what? I'll bite, because whether or not this cosplayer is a troll, there are plenty of other impressionable young cosplayers who are starting to believe that if every single part of your body does not somehow exactly match that of the drawn or animated character they're cosplaying, they're failing/not doing it justice/doing it wrong/a bad cosplayer. And I'm not about to let that belief become widespread.
It does NOT matter if your physical proportions are not the exact same as the character, whether literally (height, measurements) or figuratively ("my nose looks different"). Unless you are being paid by Disney to be one of their face characters, in which case they as your employer have some right to dictate the proportion of your weight and height to fit their princess ideas, then there is no legal, rule-bound, or logical reason why anyone should hold themselves to the standards set by any character in media. Particularly if said character is not being played by a live actor who has proportions you can actually measure, but instead is a drawn or animated caricature of a living human being. No one is going to revoke your right to cosplay because your butt isn't the same size as this character's appears to be. As long as you look good while wearing the costume, what does it matter? I don't care if the game makes a running in-joke about the size of her ass, unless you know that her hip measurement is actually listed by the (probably male) game designers as, say, 50" and yours is maybe 40" on a bloated day, you can't say that your ass is all that much smaller. Once you make the costume look like that, it's going to emphasize your hemispheres anyway.
If you're doing this for private, personal cosplay? You have even fewer reasons to cry and stress over the size of your ass. I can see where someone being paid to model as a booth babe, or someone in competition who was really unpleasantly twiggy and had no way to pad out the costume, might be a little concerned about finding ways to make their butt look bigger. But whatever your private and personal reasons are to have the costume, if you're not showing it to anyone in public at a place where decency standards and rules may apply, what's the problem? Make the costume 100% accurate and see what it looks like in a mirror. Don't forget to pose with your butt sticking out like in the center pic, it'll help the illusion a lot more than you think.
If people have personal reasons for body shaping - some of us do, let's face it! I fully admit that I'm vain and want to both drop weight and add muscle to look better in my costumes - those are personal and every individual is different. So, really, if you have a personal reason for wanting to have a shaplier ass, fine. So be it. No skin off my teeth. But if you've scoured the internet and there is NO fitness or nutrition or diet expert who can tell you that it's possible, given your genetics and metabolism, to magically add muscle or fat only to your gluteus maximus and nowhere else, then you'd pretty much better just suck it up and be thankful that you don't have a monster ass. Some of us would be all too grateful to perform a body swap and give you ours. There comes a point at which no amount of diet and exercise/training will shape you the way you want - that's when you sigh, throw up your hands, and say "oh well," and then go and make the costume anyway. Make it, wear it, make it fit you perfectly, make it look amazing on your body that you have. I guarantee that only the most shallow and jerktastic of people will still say, after all that, "yeah, her costume is amazing and she's really gorgeous, too bad her hip measurement appears to be smaller than 50" around."
Founder and President, Madison Area Costuming Society, a chapter of the International Costumer's Guild
Strange Land Costuming - www.strangelandcostumes.com