LOL not the terrain for sure and not the air pressure. I did measure the circumference
of the 2 rear tires and one is bigger than the other. I'm thinking that may have something to do with it?
You are halfway there.
Do the tires have the same size stamped on both?
Are the tires from the same manufacturer?
Those things are important.
If they are from the same manufacturer and are stamped the same, there is something that you can try.
Remove the smaller wheel assembly and pump it up to the maximum recommend pressure and set it out in the sun. Leave it there for the entire day. This will stretch the tire. When done, let some air out to your desired pressure setting. That should get them close to one another.
If they are of different sizes, you will need to replace the tires. It’s always best to buy tires in pairs. Even then, you might still have to stretch both new tires.