I know there are a dozen guys active in this thread that can answer better than I can... (but that won't stop me!
The K-frame's were the original .38 Special guns. They made -millions- of them. When the .357 Magnum arrived, it came in the BIG frame guns...the N-frames.
Bill Jordan and others urged, begged & pleaded with Smith & Wesson to chamber the rip-snortin' .357 Magnum in the .38-sized K-frame. As uniformed police officers, the idea was to practice with .38's, but carry heavy .357 Magnum loads on duty.
The K-frame magnums were much easier for a duty cop to wear on his belt day in, day out, all day long. But when folks started shooting a LOT of full-bore .357 Magnum ammo through them, the guns took a lot of abuse. And when higher velocity and lighter bullet .357 Magnum performance ammo arrived on the scene, these lighter bullet loads really
started tearing up the K-frame guns. The bulk of the propellant flame was blasting at it's peak as the shorter bullet was leaving the cylinder, and the full force of that flame was eating at forcing cones and top straps like a plasma cutting torch.
In 1980, the L-frame was released. The areas that faced the most stress from full-bore .357 Magnum rounds were all beefed up. But they designed the gun to use the grip frame of the very familiar and much loved K-frame.
So the L-frame revolver is "medium" sized, between the K and N frame, but the grip is the familiar K-frame. Many of us feel that the L-frame is just exactly right. And yes, the L-frames do have full under lug barrels. But a few K-frames and some of the N-frames have those also.
When you've only worked with a bunch of K-frames and an L-frame or two... every N-frame feels massive in comparison.