/... The Six series guns are what I would say between a K and L frame. Ruger sized their revolvers in between S&W frame size. I think Ruger made that decision to offer something different that was not already out there with the competition.
SP101 = between J and K
Security Six = between K and L
GP100 = between L and N
Redhawk = between N and X
The Model 19 was designed in an era where departments used .38 Special for practice and then carried .357 Mag, but there were numerous lawsuits that alleged that officers who practiced with .38 Special were under trained for the use of .357 Mag for carry on duty.
The end result was that departments started using .357 Mag for practice, when the Model 19 wasn't really designed for a steady diet of .357Magnum loads. That shift in the ratio of .38 to .357 loads was a contributing factor to Model 19s having issues with cracked forcing cones.
That became an issue around 1970, the point when Ruger was making the jump into the Law enforcement revolver market and they no doubt saw it as an opportunity.
Thus, I think Ruger's intention with the "Six" series DA revolvers was to make them just large enough to stand up to continuous use of .357 Magnum, while not losing the nice handling qualities of the K-frame .357 Magnum revolvers.
They were successful in that regard as the Six series is just a bit heavier than a K-frame, but not to the point of being al that noticeable.
The same logic applied with the SP101 series compared to the J-Magnum frame .357 revolvers. The SP 101 is about 5 oz heavier than a steel J-frame with the same barrel length, but holds up to a steady or frequent diet of .357 magnum loads much better than a Model 60.
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Below is a 3" Model 13 along side a 2 3/4" Speed Six. As you can see, the Ruger's frame size is only very slightly larger than the K-frame used on the Model 13 and Model 19, but it is heavier around the barrel and forcing cone and carries more weight in the recoil shield, which mostly adds weight, not strength. The Ruger also does not incorporate side plate, which gives it a solid frame, which added some strength. Finally, the lock work parts in the Ruger were much more robust and almost never broke.
The photo on the top has both revolvers with Pachmayr Compact grips, while the latter have signature grips that make the grip frame seem much bigger than it is: