It is a K frame, I stand corrected.
Here's an interesting post from a guy on a Smith forum.
They are actually quite close in size and a 617 will actually weigh MORE than a 686 with the same barrel length. As for why, smaller bores in the cylinder and barrel means that those parts weight more.
As for the key difference, the cylinder in the K frame is 1.45 inches in diameter and the cylinder on the L frame is 1.56 inches in diameter. Because of this the window in the frame on the L frame is 0.11 inch "taller" and the barrel is 0.055 inch further away from the center of the cylinder. Finally, the stub that threads into the frame on the L frame is a bit larger than the stub that threads into the K frame, so barrels cannot be swapped between the 2 frame sizes.
Beyond those differences due to the larger cylinder, the K and L frame are basically identical, length of the frame windows are the same, the top strap has the same width and thickness, and the area under the frame window is basically identical. Also, the K and L frame use the same grip frame.
Basically, the L frame is a K frame that was enlarged just enough to correct the slight weakness that the K frame demonstrated with the 357 Magnums that were used with light bullets loaded to maximum pressure. By raising the barrel be 0.055 inch in relation to the center of the cylinder the flat on the underside of the barrel to clear the gas ring on the cylinder was eliminated in the L frame. At the same time they enlarged the barrel stub that extends into the frame by IIRC by 0.02 inch. The result was no more split forcing cones in the Magnums in a revolver that wasn't much heavier. BTW, the greatest weight gain in most of the L frames is due to the full length barrel underlug that is featured on most L frames. However, the 686 Mountain gun which features a Tapered semi lugged barrel is listed at a weight of only 32 ounces, which is actually 2 ounces LESS than the weight of my 4 inch tapered barrel model 67-1 K frame. However, I have a sneaking hunch that when that 686 Mountain gun was actually weighed it didn't have the grips mounted because I would expect that with the standard Hogue rubbers it will actually weigh in at 34 to 35 ounces, basically the same as my 67-1.