Well, then famous pro bodybuilders would be a good case study (there are heaps of photos of them with extremely low and normal BF %). Say, Dorian Yates. A quick google image search showed little difference. In-contest-form his simply face looks haggard, rather than defined.
Have you never seen someone going from overweight to normal or thin and see their face shape change? Honestly? Come on. Obviously it's not going to make as much of a difference on those with poor/undefined bone structure but but the face carries fat, and if you lose the fat, you will see more definition. (but not become a supermodel
)
As far as I understand it, weakjawbrah has normal cheekbones (rather than flat like class3s often are). But is not as thin as a model and that's where part of the cheek concavity comes from - good cheekbones AND low body fat. I've got a friend who is a boxed and every time he drops weight for competition his cheekbones appear more prominent and he has more cheek concavity
Casting calls for plus size models almost always state 'slim face'