[Fat Charlie]

I can see low pressure with a car sitting on top of it, allowing deformation and reduced diameter. Higher pressures really shouldn't stretch the steel belts past nominal size. I'd think higher pressures would increase resistance to deformation (=LRR) without an actual increase in diameter.

This doesn't account for people like Ford specifying really low pressures in their Firestones for a cushy ride, but if reasonable people set the spec, going to max sidewall or beyond shouldn't affect size.