Howdy
It's really a lot simpler than all that.
At least in S&W land. In the early days of Tip Ups and Top Breaks, frame sizes were numbered. 1, 1 1/2, 2, & 3. We can go into that at another time.
With the coming of the models with side swinging cylinders (Hand Ejectors) S&W gave the frames letters. M, I, K, and N. Let's talk about M in a minute. The other three were the sizes that could house a cylinder that could accept six cartridges. I for six 32 caliber cartridges, K for six 38 caliber cartridges, and N for six 44 caliber cartridges. It's that simple.
That is not to say that five shot versions of some of these did not exist for larger cartridges, they did. But in the beginning I, K, and N size frames were developed to house cylinders that had six chambers for each of those three calibers.
Later, the I frame was stretched to accept longer cylinders that could accept five 38 Special cartridges, resulting in the J frame.
The tiny Ladysmith was the only revolver made with the M frame. A seven shot revolver chambered for the 22 Long. Only made from 1902 until 1921.
Pictures are worth 1000 words because words are meaningless without comparison photos department:
Top to bottom:
N Frame 44 Special Triple Lock
K Frame 38 Military and Police
J Frame Model 36 (Chiefs Special)
I Frame 32 Regulation Police
M Frame Ladysmith
Camera Shy:
L Frame Model 686. This size is between the K and N in size. It was developed to increase the diameter of the cylinder slightly from the K frame. This was done so 357 Magnum revolvers could be made without a relief cut on the underside of the forcing cone. The relief cut on the forcing cone was often a problem because the forcing cone could split with some 357 Magnum loads.
Also Camera Shy:
X Frame. A ridiculously over sized frame developed for ridiculously overpowered revolver cartridges such as 454 Casull, 460 S&W Magnum, 500 S&W Magnum.
Z Frame. An even more ridiculously over sized frame for the Governor; 45 ACP/45 Colt/410 Shotgun cartridges.
X Frames and Z Frames are not allowed in my house.