Noticeably more Muscle on one leg than the other?
It's normal to have a small amount of imbalance on one side vs the other. Significant imbalance can be due to injury or neurological issues on the weaker side. If either of those apply, it's also common to have knee pain on the stronger side as it is trying to make up for the weaker side. Even after recovering from injury or neurological problems, it can take months for the weaker side to catch up, even longer without physical therapy. 

Have you ever hurt your knee, leg, foot hip, or ankle on the weaker side? Any lower back problems? Sometimes nerve impingement can manifest itself as weakness without pain, but it's more common to have both.