In order to get a thicker branch on a bonsai fig it would mean getting the branch to grow faster and have more leaves than other branches on the tree. Another way is to fuse several branches together to achieve greater thickness.
This Ficus ‘ExoticaJS’ has been in training for only a few years from rough pre-bonsai stock. It is an excellent material for bonsai and it is not often available for sale.
Periodically I defoliate many of my bonsai figs. Doing this produces smaller leaves and makes the tree more showable. It also allows me to see defects or problems in the tree more easily than if leaves are covering the tree.