What causes that certain look some people get after gastric bypass surgery?

I don’t know for sure what you are talking about, my guess is either it is fat deposits in unusual places, or poor posture from the weight they carry around, or if they are women possibly they had PCOS from a young age from higher sugar and insulin levels and have the typical body structure seen when that disease occurs during a teens growing years (but not always).

Often it is that their skin is saggy and doesn’t seem to “fit”.

I would go further than @clod it’s not that their skin doesn’t seem to fit it’s that their skin quite literally does not fit. The skin has a little give in it but not much once someone have grown to a large size and then lose a lot of weight quikly they are left with way too muvh skin. It hangs off them in folds and often requires plastic surgery to remove (in the UK the NHS will often pay for the gastric band but won’t pay for plastic surgery). The arched humped back is probably due to the posture they had when they were fatter and physically could not stand in the correct posture with all the extra weight.

The case you describe is not common of weight loss surgery patients. I know several post-op gastric bypass patients and none of them have the symptoms you indicate.

However, it is possible that poor nutrition leads to the less-than-healthy skin.

I agree with @Lightlyseared about the likely cause for the hunch-back.

I have had weight loss surgery,and I recently went to my first post-op support group. Nobody in the group had the appearance you describe. Everyone is monitored on a regular basis for vitamin levels, so that may explain why everyone’s skin looks normal, not gray.

I was going to add that the one person I know who had the band, she lost over 100 pounds, looks fantastic. She is a beautiful woman. She had some of the extra skin removed several months ago, but even before that she looked good.