We often assume that our right and left side limbs are symmetrical; but it’s not unusual to have differences in size or strength based on how you use each limb — or differences in neurological wiring. Without even realizing it, you may place more load on one leg during your daily activities at work or at home.
Assuming you don’t have any medical issues that could explain the size difference, the most likely reason is that you’re performing double-leg strength training exercises, like squats, leg press, and leg extension. If one leg is stronger than the other, it’s going to bear more of the load compared to your weaker leg. Because hypertrophy (an increase in muscle size) depends on the training stimulus, your stronger leg is getting a better workout, so it’s getting stronger and bigger while your smaller, weaker leg is, in a way, going along for the ride.
To get your smaller leg to step up and get into the game, swap out your double-leg routine for single-leg training until you achieve a greater level of symmetry in terms of strength and size. For example, instead of squats, perform forward lunges or TRX ® suspended lunges; do leg extensions one side at a time; perform dumbbell step-ups.
These exercises will force your smaller leg to perform more work. As this leg gets stronger, gradually increase the workload; it’s reasonable to expect this leg to catch up to the other in terms of strength and size.