Never had the opportunity to take physics in school. It's on the bucket list, but will have to wait for retirement in a couple more years.

I've been wondering about this for a long time, and a whole lot more recently, spurred by putting 28mm tires on one of my new frame builds.

My basic question is: why do larger tires, (internal volume?), take less tire pressure?

For example, when I used 23mm tires, I ran them at 120 psi. Switching to 25mm tires, I dropped that to 110 psi. Now with the 28mm tires, it has been recommended to me to go with 85-95 psi, (I was lamenting that I didn't get the comfort I expected, probably due to riding with 110 psi in them). To take it to the extreme, automobile wheels run at 35-32 psi.

In these four examples, each increase in internal tire volume take less air pressure. Why is this?