With all black powder era cartridges,the gr in powder is how much the case will hold,thus a .45-75 or a .45-90 case will be bigger than a .45-70(not much larger with .45-75 however). By the way,Guess I've led a sheltered life as I havn't heard of a .45-75.
The 45-70 technically is called the .45 2 1/10" because all of the older cartridge cases were loaded with various bullet weights and black powder charges. As time progressed the manufacturers started to use the following nomenclature of 45-70-500 which indicated the caliber/black powder charge in grains/mass of projectile in grains.
The 45-90 is technically a .45 2 4/10th and has nothing to do with "90 grains of black powder" as today you can rarely put 90 grains in it.
Because all of this is so confusing it is generally easier when people refer to the original nomenclature of cartridge caliber and length.
Yes there is a 45-75 but it isn't the same as the 45-70 it is a totally different case. Unfortunately no one is manufacturing them right now so they have to be formed by using .348 WCF brass and it is a pain in the neck.