I have a similar problem. Right leg is much smaller than my left, primarily the hamstring but the quad as well.
I'm certain mine is from an ACL repair when I was 30. Surgeon took a tendon from my right hamstring to replace my right ACL. I believe this led to an imbalance in my glutes as well.
I tend to drift to one side when squatting and and when doing split squats, my right glute shoots out while I'm able to stay perfectly straight in the torso on my left side.
I don't know there's too much you can do if but here are some things I've done that helps:
- unilateral exercises, do the weak side first and limit your reps on the strong side to what you did on the weak side
- do one to two extra sets for the week side each exercise
- do a set on the weak, then the strong and then a few more reps back on the weak
I also have some lower back issues on my right side (right where the lumbar meets the glute medias), probably from the weaker hamstring and glutes on that side. So I tend to do a lot of glute work, reverse hypers are amazing for my back and supermans are good as well.