The leaves on my banyan bonsai were small but as the plant grew taller, the leaves enlarged even after defoliating. Now they are 10 inches long and the plant is just a feet tall.
Do not defoliate too often because that hinders the plant's growth. Just prune and defoliate in the end of winter so that your plant has full potential to grow during growing season. You can defoliate again before rainy season to have more leaves. That will allow more roots to come out. More roots, more hardy the plant.