There are two basic but related questions; How do I increase the trunk size and how do I increase the trunk taper? This point is often quite unclear to bonsai beginners. They often ask how to thicken the trunk when what they really wish to accomplish is more taper.
Anything that you can do to increase the growth on the tree will increase the trunk size. Naturally this takes time but the key fact is that the more leaves and cholorophyl, the faster the trunk will fatten. Using a larger container, good fetilization and not trimming the tree will accomplish this goal. However, often the trunk is actually of good size but the trunk does not appear large due to lack of taper. It is taper that gives the tree a big boost in achieving a mature look and not the raw size of the trunk.
One way to get a large but well tapered bonsai is reduction. All the fabulous trees that you see in my book, on the internet and in show publications are large trees that have huge bases and lots of dramatic taper. These have had years of strong growth in a large container or the ground and then were cut back to a branch which formed the new apex. This new apex was then allowed to grow strongly until it was about half the thickness of the trunk immediately below it.
This process is repeated at intervals introducing directional movement to the trunk as well as tapering segments. Once the tree has gotten perhaps half way to its final design, attention is directed to finer details like branches and silhouette. Wiring and grafts and other techniques are then used to correct deficits.
This process takes years outdoors in tropical climates. Indoors it will take many, many years.
Another technique to create a large trunk is to fuse smaller trees together. Smaller figs are placed together, the outer bark is protected and the trees are wired to each other. With time they will grow together and form a larger tree. See Ficus fusion techniques.