"I have always just done running, jogging, cycling etc on legs. While working out upper body. Now I am trying to gain mass on my legs. I'm 77kg and I can do 15 single leg squats on my right leg with an additional 20kg. I don't have the balance to do the added weight with my left leg, I just tip over. Help?" - Joe.
This is actually normal.
Everyone has dominant arms, eyes and legs. Even dominant ears that you hear better out of.
BUT if you really want to try and correct the problem there is a solution.
#1. When exercising do DOUBLE whatever you are doing on the left leg compared to the right. So if doing single leg squats do 15 on your right leg and 30 on your left. If you are falling over use a wall or friend for support.
#2. Whenever you get the chance to do something with one leg over the other, use the left leg in your daily routine. This will require some memory to do so.
#3. Do yoga to improve your overall strength and balance.
#4. Attach a small leg weight to your left ankle and wear it 8 hours each day for a week. After the week is over do a series of strength and balance tests to see improvement.
#5. Consult a professional bodybuilder who has encountered the same problem and find out how they fixed it. Always handy for extra advice.