I've posted on it in depth before, but there's more to it than just the bullet weight. It's the 125 gr bullets that get the bad press for forcing cone cracks, while 110 grain bullets are never implicated, and heavier bullets are also not associated with forcing cone cracks.

The big internet myth, a carryover from the older gun rag myth, is that the short 125 gr bullet allows gas to by pass it in the cylinder and pre-heat the forcing cone before the bullet gets there. Thermodynamically speaking that theory doesn't wash as there is not enough plasma or time involved to heat the forcing cone significantly before the bullet gets there. And if that were true, 110 grain and 90 grain loads would have that problem in spades.

The critical difference is the powder used and the amount of powder used in 90 and 110 grain loads, versus 125 gr loads and 158 gr loads.

The light 90 and 110 grain loads do not use slow burning colloidal ball powders, instead they use medium burning powders. Those powder charges are lighter by weight and are generally not large grain abrasive colloidal ball powders.

Slow burning colloidal ball powders are the norm for 125 grain and 158 grain loads, the difference is that the lighter 125 gr load will use 21-22 grains of powder, compared to 15-16 grains for a 158 grain load. That equates to about 40% more plasma and partially burned powder flowing through the forcing cone with each shot.

That extra 40% greater powder charge is also additional heat working on the forcing cone behind the bullet, and it's 40% more mass and 40% more abrasive partially burnt powder flowing over the forcing cone. Over a large number of rounds, it makes a big difference. Even without the loads running hotter and accelerating the forcing cone erosion, it's still 40% more powder and plasma mass flowing over the forcing cone with 40% more erosion per shot. It's harder to calculate how much acceleration the extra heat adds, but the 125 gr loads using slow burning colloidal ball powders probably produce twice the erosion per shot as a 158 gr load using the same powder.

Given the very high round counts usually needed to get enough erosion to initiate a forcing cone crack in a K frame .357 Magnum with 125 grain (which still occurs in an exceedingly small percentage of K frame .357s), you'd need twice the round count to get the same effect with 158 gr bullets. By that time the revolver will have almost inevitably been fired with at least some 125 gr rounds and a if a crack occurs they'll get the blame. When it's really the colloidal ball powder doing the damage.

The 125 gr bullets however took the rap as they came along about the same time that colloidal ball powders started to be popular in the .357 Magnum (and of course they use more of it). And, that occured at the same time police departments started getting sued for under training officers and switched from practicing with .38 Special and reserving .357 Magnum for duty use, to using .357 Magnum for practice as well.

So you now had 125 gr bullets powered by large charges of colloidal ball powders being used all of the time in Model 19s that were designed to primarily shoot .38 Special with occasional firing of .357 Magnum loads - which when designed used medium burn rate flake powders.