Here is a bit of a size comparison of various .45's for which I load. From left to right, we have the 540 grain "Creedmoor" bullet I use for long range competition. Notice it is bigger than the entire loaded .45 ACP round right next to it. Next is a .45 Colt, loaded per original Frankford Arsenal specs with a 250 grain bullet and 40 grains of FFFg. Next is a match load for the .45-2.1" (.45-70) with the Creedmoor bullet. Next up is a modern smokeless powder .45-70 load reserved for my Marlin and Winchester lever guns, featuring the 400 grain RCBS gas check bullet at over 1,900 fps (ouch...). Then comes the .45-2.4" (.45-90), the caliber in which I ordered the new rifle. It's loaded with the 400 grain Lyman bullet over 85 grains of Swiss 1.5 Fg for about 1,300 fps. Next up is the same .45-2.6" (.45-100) shown above, with the Creedmoor bullet over 90 grains of Swiss 1.5 Fg for about 1,300 fps. Last is the mighty .458 Winchester Magnum, with the 500 grain Hornady soft point getting about 2,150 fps (double ouch...). The .458 looks weird because I seat the bullets out long for use in my Ruger #1, which has a very long throat. Since it's a single shot, I don't have to crimp - that's why the cannelure is so far out. This allows me to approach .458 Lott ballistics with the shorter case.
So, the .45-2.4" is a good, happy medium in the black powder cartridge world. It hits about the ideal capacity/bore ratio for a .45 caliber with black powder.