I read a training tip that pro bodybuilder Bob Cicherillo said to be a good rule of thumb: In for out, and out for in. In other words, if you move your hands or feet in, you’ll generally hit the outer head of the target muscle. If you move your hands or feet out, you’ll generally hit the inner head.
That seems to be especially true with leg work. If you widen your stance on calf raises, it forces you to come up on the inside of your foot, better affecting the inner head of the gastrocnemius. If you use a stance with your feet close, you tend to roll out on the outside of your feet, which affects more of the outer calf head.
MRI studies show that effect on quad work as well. Feet close on squats hits more outer quads; feet wide hits more inner quads and adductors. If you’re looking for more sweep on your outer quads, at least as much as your genetics will allow, use a closer stance.
MRI studies also show that turning your feet slightly inward on leg extensions hits your outer quads more as well.