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

I was watching some TV shows about seriously obese people who got gastric bypass surgery and lost hundreds of pounds.

I noticed that after they’d lost weight, they all had a certain look that’s hard to describe. Kind of grayish skin, arched/humped back. What causes this? My guess was poor nutrition for the skin tone, and having all that weight for the posture. I’m just curious about this, so I’m wondering if anyone knows.