Anybody help me with Bonsai?

I have two bonsai of peepal tree. (Ficus religiosa). they both are about 15 years old. my problem is that their leaves size is very big about 8 cm across. i tried different methods to reduce the leaf size like defoliation, cutting the leaves in half etc. but could not succeed. can anyone help me with reducing the leaf size and increasing density of foliage.