Noticeably more Muscle on one leg than the other? One of my legs is noticeably more muscular than the other.  I am also more prone to knee pain in the more muscular leg.

Could this be related to my frequent (I ride 5+ days a week) cycling habit?  Are there any exercises I can do to even this up?  I have to walk/climb stairs a lot at my job...

Should I even care about this?  What habits can I break that might cause this?
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.