I seem to have quad muscle imbalance.
I'm not sure how "properly" built cycling quads are usually proportioned, but it seems that mine are a bit out of. My inner quad is noticeably less built than the middle and outer muscles (Vastus Medialis is less built than my Vastus Lateralis and Vastus Intermedius/Rectus Femoris). I know that the inner quad muscle is smaller than the rest, so maybe I'm overreacting. I'm not noticing any power issues, it just seems a bit odd.
Is this bad? Should (can) I work on this at the gym? Does this have anything to do with my knee pain/tightness on 40+ mile rides? Does anyone have any information about this?
Pretty sure that's normal unless I am not understanding you correctly.
The more pigeon-toed(hip medially rotated) you pedal the more you would shift the load and the muscle development toward the lateralis and away from the mediallis.
Doing leg extensions with the toes angled out(hip laterally rotated) should develop the medialis somewhat preferentially.
Should you worry about any of this? I don't claim to know but I would doubt it.