Let me start by saying I really like the vast majority of what you produce, as well as the concept at work here. Cilestia being fat off of cake is funny to even say and more amusing still as an image. Cilestia + cake is really common but I don't remember seeing this idea executed in exactly this way before.
However the image itself doesn't really do much for this concept. The biggest weakness holding this image back is the lack of proper focus. Every other image you produce has a fantastic usage of lines lighting and character expression to draw the observer into whatever position and mindset you want.
The lines in this image draw the viewer to the arch of her back when it seems like the main point of the joke is her expression and the text on the scale. The size of the thought bubble takes up a large amount of the image and means that the arch of her back is at the center, further emphasizing it.
Also she just doesn't look fat in this image. I know slenderness is part of your style and you use it well most of the time but here it works against you. Even if its just a subtle chub there needs to be something in this image that justifies her reaction more than the text on the scale.
The background is the same color as your subject. You know better. Not being hateful. You genuinely have much better technical skill than this in most of what I see in your gallery. Dynamic lighting, emotive characters and focus that is worthy of emulation is all over your work. This piece seems to strangely avoid everything you are good at with the exceptions of interesting situations and well executed facial expression.
I hope this wasn't too harsh and I certainly don't want to make you feel discouraged. I would be proud if I made this image, but you can and have proven that you are way better than this.
Actually there's some pretty good evidence for it being "largely" genetic. Sure it's not EXCLUSIVELY genetic, but it does play a role.
"Twin studies have shown a heritability rate of 56%."
"Association studies have been performed, studying 128 different polymorphisms related to 43 genes including genes involved in regulation of eating behavior, motivation and reward mechanics, personality traits and emotion."
That only proves the predisposition is heritable. The genes just increase the risk of getting aneurexia. The twins in question live together and are subjected to the same influences. 56% is relative low considering that.