In dealing with a fig that has too heavy a root there are several ways to handle the situation. One, is to simply use soil and moss to partially or completely cover the thick root.
Another solution is to cut the large root completely off. After removing the root seal the cut with cut paste and cover lightly with some soil or sphagnum moss. Usually the cut root will sprout and replace the heavy part with a new and thinner root in much better scale than the original. Cutting off one large root on a healthy tree should not prove to be harmful to the bonsai tree.
Yet other ways to handle the heavy root is to split the root or cut the root in half lenghtwise.
All of these will result in a root that is proper scale to the trunk and design of the tree.