Even though the case of the .357 Magnum is only marginally longer, you have to realize that the case capacity is used much more fully than with the 38 Special case.
Most 38 loads only fill the case about 15-30%. With a bulky powder like Trail Boss, you may fill the case more, but let's say you are using Clays or 231 with a light HBWC load...you can charge the case almost 7 times with a recommended load before it overflows. Because of this small charge volume relative to case volume with the 38 Special, it can be a dangerous caliber to reload. Double charge the 38 Special (you'll have to look hard to see the powder in the case even with twice the listed amount) and you are loading this cartridge to dangerous pressure levels.
The 357 Magnum is designed to use more of the case capacity, hence much higher pressures etc, and any double charge is usually very evident as the case overflows with powder.
Unless you actually reload these calibers, you'll not have these observations before you.