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
If a bonsai needs to have a heavier branch than it must be allowed to grow faster than the other branches.
Thickening will occur in direct proportion to the number of leaves photosynthesizing on the branch. Keep more leaves to produce more energy and more growth to thicken the branch. After the branch has enough thickness it will need to be shortened back to proper length for the design and then twigging density can be accomplished.
Branches also can be kept on the trunk to accomplish trunk thickening and later removed. Thickening only occurs on the trunk below the branch and not above it.
Branch on right needs to be heavier so more leaves are left on the branch
Branch 1 and 2 are kept to thicken the base of the trunk. They will likely be removed at a future date when the trunk is thick enough.