I am curious as to the different frame sizes. It would be easier as a manufacturer to use the same size frame. That's what they did with K frames, the .357s had less gap between the front of the cylinder and the frame than the .38s because the cylinder windows were the same, but the .38s had shorter cylinders (making a larger gap from the front of the cylinder to the frame).
Part of the reason is timing. Consider this, say you're making the J frame .38s for 35-40 years or more, and then somebody figures out how you can stretch the frame enough to fit .357 into it, and make it at a marketable price point. Cool....
Now, do you STOP making .38s on the standard frame? snd have the tooling and jigs unique to the .38 frame sit idle, gathering dust, rusting, and making you no money???
OR do you continue to make .38s on the .38 frame (which are still selling well) and then ALSO make .357s on the longer frame, keeping the market you had and have for .38s and now adding .357.
Now turn that around a bit for the K frame. You've been making the K frame size since the dawn of time (well 1902, anyway) in .38 special. The frame ALSO happens to be large enough to fit the .357 when it comes along, so you can make .357s on that frame with minimal changes.
Another point to consider, you mention the gap between the front of the cylinder and the frame, and how the shorter .38 cylinder has a larger gap. This is true, but it doesn't matter. The size of the gap between the front of the cylinder and the frame doesn't matter for anything other than visual aestheics. What matters is the BARREL/CYLINDER gap, and that is kept to standard tolerances with a short cylinder simply by having the barrel extend deeper into the frame window. An obvious example of this is the N frame guns in .45acp. Very short cylinder, compared to .357, .44Mag or .45 Colt. The barrel extends deeper into the frame window, up to the front of the short cylinder, keeping the barrel/cylinder gap within standard tolerances. The short cylinder is a "long" way from the front of the frame window, but its the normal distance from the rear end of the barrel.
Doing it that way means you don't have the change the frame at all for a short cartridge with a short cylinder, all you have to do is cut a few more threads on the barrel shank where it screws into the frame. Simple, easy, and cheap compared to altering the frame.