We have three foot tall humanesque beings swinging axes and great swords with the same power and speed as seven foot Goliaths of men. Those same Goliaths run at the exact same speed as the three foot little people. People with horns/ears and tails. People with green/purple/red/blue skin.
I understand that the best of fantasy stories are grounded in reality, but the whole "it's not realistic" thing was kind of thrown out the window before 2.0 even started.
Plus different bodies do hold onto fat differently. I've seen more than my fair share of wrestlers and military folks that were perfectly athletic and could perform their duties excellently, yet they maintained some pudge (particularly around the belly/lower belly). Some folks just have permanent pudge. Especially if they're beer drinkers (>__>) Sorry folks: as much ale as gets drank in this game you can bet there would be beer bellies walking around Eorzea.
As for the the slider I'd like to see? Honestly I'd agree with having a "fat" slider and a hip slider. From a perspective on female characters (as a woman), it's incredibly rare to rock C+ breasts yet have a size 0 waist. Some of the models have better limitations to reflect this (like the Au Ra. Even at max their breasts are still proportional to the rest of their bodies, and thus remain pretty small), but several of them look...odd when maxed. Like their backs should be snapping in half (Hello Minfillia!). It's also super rare to have a heavy chest but not have hips. Not even the Roe ladies of this game have hips that actually match their chests. While I'm not a fan of Black Desert Online that was one area they actually did very well: the girls actually looked proportionate to their body builds. My little witch/wizard girl actually had hips and thighs.
Granted that goes against my whole "realism" argument. The characters in this game are all gorgeous as they are, but I do agree if we had some more control over their proportions it would be nice.