You can’t scale or transform the armature, as the bones snap into their original orientations when animated, resulting in the stretched legs you see with your results. It’s a bit difficult to get decent results with models that don’t fit the rig as well, but I believe most people just weight the upper leg bone to the lower portion of the torso and the upper part of the leg (if the legs are long enough), with the lower leg weighted to the rest, or if the legs are too short, the upper leg unweighted and the lower leg also partially weighted to the lower portion of the torso.