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.
The red circle shows three small branches that are coming out of nearly the same spot on this tree
The three branches were fused into one and created a branch that is heavy enough for this trunk
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.
Ficus ‘ExoticaJS’ seen with all its leaves
After defoliation the structure of the tree can be seen and it is apparent that the right lowest branch needs thickening and more sub-branching while the left lowest branch is too thick and needs to be kept from growing too much