You have to remember that the cases/ammo of the time was loaded with black powder. As you figured out, the first number is the cal, and the later the amout of Black power. In regards to the 45-70, the actual working pressure of the round is only around 18,000 psi, and the 45-120 just a little over 20,000psi. As for case length, yes themore powder, the longer the case since these were straight wall cases.
To take this into the now future, the only round that was converted to smokless powder was the 45-70 goverment, but still loaded to the old BP working pressures. Funny part is now with the use of modern steel and smokeless powder, a rifle in say 30 win mag has working pressure of around 50,000 psi.
Bottom line is the steel of the time would only safely retain around 25,000 PSI before blowing apart ,and this really limited the amount of BP that could be used.