The over-all length of the cartridges is the same. The 45-90 uses a shorter lighter bullet in a longer case. Being black powder, both cases were filled clear to the top with powder. I ran both through an 1895 Marlin in 45-90 and could not tell the difference in accuracy with casual plinking.
In 1952 I shot a 45-90 high grade '86 half octagon, 2/3 mag, fancy wood against three like new military Trapdoors all using blue label Govt surplus black powder ammo. I forget the 45-70 bullet wt . The 86 outshot all three 45-70 Trapdoors. We switched guns and the results were the same.