Why does a person carry more weight when sitting down?

Assuming by “carrying” we mean stress on the backbone, we can think in terms of three types of stress on the backbone: tension, compression, and bending.

Insofar as pure tensile and compressive stresses are concerned, they are a function of axial loading.

Bending stress is a combination of tension and compression. For concave upward bending (smile) you have compression at the top and tension at the bottom.

I would think that all vertical positions would involve compressive stress. But since the backbone is not perfectly vertical (see diagram below) vertical positions can also produce tensile stress and bending.

All of this makes it very complicated because you need to isolate what type of stress is under consideration. However, we can make some general observations.

Any vertical position will maximize compressive stress on the backbone. So standing and sitting straight up should maximize compressive stress. Why sitting down is more than standing up is not clear. Perhaps it has to do with concentrated stress that the reaction force of the seat imposes on the bottom of the spine (tail bone). Or perhaps sitting down causes more curvature of the spine, though I’m not sure.

Any horizontal position will minimize compressive, tensile and bending stress. So lying down should be less “stressful” on the backbone than all the other positions.

Sitting and leaning over would probably be the most stressful since bending of the backbone is maximized. This results in both tensile and compressive stress on the backbone.

Hope this helps.