Interestingly - all your weight is on the harness, but the wall is still what makes the difference. Suspension trauma is basically a bloodflow issue, so resting against a wall lets you keep your feet elevated and your quads active, stopping blood from pooling. The catastrophic case for suspension trauma is hanging limp and unconscious, since it lowers your feet while stopping muscle use or even fidgeting to boost circulation.

The vast majority of your weight is in the harness, but there is some portion of the force vector that's going into the wall as well.

But you're right, the weight distribution isn't really what matters in this case.

If the belay stance is a ledgeless hanging belay and the climber above you is slow, you can be hang as much as you end up wanting. I'll end up putting a hip into the wall to give my feet a rest.

But even during super long belays, it's unlikely for the belayer to be motionless for even ten minutes.