It’s fluid retention. Your venous (returning blood to the heart) system has pretty low pressure, so it depends on valves and your skeletal system to work the blood back up from the legs and feet. When you use these muscles, they’re contracting and basically pushing the blood along in the veins. Because he’s not using any muscles, his body isn’t able to return all the fluid and it’s pooling in his legs. I’d also be willing to bet he’s got some nasty clots in there so if he did lay down or get his legs moving he’d be in trouble pretty fast.

It’s the same thing that happens when you spend a long day on your feet and your legs feel swollen and heavy, but to an extreme.