I think most athletes have a disparity. To increase size, you need to overload the muscles. Strength training will work but you need to isolate the leg you want to build. The other issue is neurology. The weaker leg may not be as responsive. For example, I do one legged pedaling drills. My left leg is weaker and initially when I started doing these it was hard to be smooth and the leg would tire more quickly. Now after weeks of work, my left leg is equal to my right. If you want to challenge yourself on the bike, do one legged drills by uncleating one leg and keep a smooth pedal stroke. For more challenge, do opposite arm opposite leg drills meaning hold the bike with your right hand (release the left hand) and pedal with the left foot. You do not need a lot of power or cadence and a slight uphill grade will help. One arm holding and opposite leg pedaling is difficult.