When you stand your feet catch all the weight. That is why your feet hurt when you stand for long periods of time.
When you sit, your back is supporting all the weight. It also has to do with how you sit. Different postures don't carry all of the weight properly. Making it so that it hurts worse in different spots.
You body total weighs lets say 180 lbs ( example). However, your legs don't make up all of that. So your back sate and support approx 100lbs or so. ( I'm not a scientist). With no support it can cause pain.
That's why they have support rugs for standing and support chairs and cushions