Originally Posted by Doge

Geometry.
Pretend that the contact area needed to be the same for a 20mm and a 25mm tire.
The 25mm would have a rounder contact patch as the downward pressure increased, there was a wee bit of casing on the side - and forward and rear of the contact point.
The 20mm would contact and there would be less casing on the sides to create the same contact area. If contact area is to be the same it would have to deform more case in front and behind the contact point. That would mean more total drop, more case flex/deformation.

Under the same pressure thinner drops more and deforms the case more.

To me, depending on road, that does not mean a whole bunch. If you don't like drop, pump up the PSI.