The difference in weight between a 4" k frame (36 oz) and a 3" l frame (36.8 oz) is .8 oz or .05 lbs. The weight difference between a 2.5" k frame (30.5 oz) and the 3" 686 l frame is 6.3 oz or .39 lbs.
There is little to no difference in accuracy between a 4" and 3" revolver. So apart from very special applications i.e. competition, the difference in weight between the 4" k frame and 3" l frame is negligible. The difference in weight between the 2.5" k and the 3" l is worth considering if you intend to carry a lot (the k frames sleeker design, we're talking mm's by the way, slightly aids in concealment) but less than a half pound of weight is not worth chopping up the l frame in my opinion. Especially when you consider the l frames design was intended to shoot magnum loads, a much more effective round than the .38 spl, without potentially damaging the gun. When you get into longer barrels, i.e. 6" the difference is more notable but those guns are not intended for carry so much as hunting and range use. Hope this is helpful.