Arnold Schwarzenegger and Franco Columbu used to experiment in the gym to find ways to fix weaknesses. One thing they discovered was that doing squats with a wider stance helped build the inner thighs (check out page 123 of Arnold’s autobiography Total Recall for more details).
This is one area that I’ve been focusing on over the past couple of years with a healthy diet of wide-stance squats and sumo-style deadlifts. Wide stance, however, does not mean “edge-of-the-rack” wide – many powerlifters who adopt that stance to reduce the distance of travel end up with a hip replacement down the road. I’ve found that a “semi-wide” stance is just as effective with far less stress on the hips. I’ll talk about it more in my upcoming lecture on injury prevention at canfitpro 2019.
Take a look at the video. Notice how my feet are positioned just outside shoulder-width and slightly toed out with the knees tracking over the middle toes. Try doing squats like that if you wish to work your inner thighs.