275 is one aspect of the tire diameter and the other is the profile as with a 70 profile. The 285/70R17 are 1/2" larger in diameter than the 245/75R17 tires. The difference in height of the axle and radius of the tire is only 1/4".
Sidewall strength varies greatly between tires. The Nitto tires I bought are 3750@80PSI whereas the stock were rated at 3195@80 PSI and the load capacity is what I care about. If a tire manufacture states that a tire will be OK on a 7.5" to 8.5" rim then that is good enough for me.