Malnutrition. I know there are larger gliders out there, but those are clearly OVERWEIGHT gliders. What is the first one even eating? Looks like pretzel sticks. It's easy to get gliders to get a little over weight--they are like kids. They tend to like sugary, fattening things (mealies, yogurt, juice, etc). Mine try to eat ice cream any time I'm eating it. That paired with a small cage/no wheel (lack of exercise opportunities) could probably make a glider quite fat. Processed foods are bad, and I've totally seen Youtube videos of gliders eating Cheetos and other nasty things.
Those poor gliders though... I bet their owners either aren't informed of proper glider nutrition, or else just don't care. :[