Ficus virens with a nice thick lower trunk 

There are several ways to thicken the trunk of a bonsai. One way is to take aerials that are near the trunk and pull them over to touch the trunk and hold them tightly in place. Over time the aerials will fuse to the trunk enlarging it, as well as creating improved surface rootage/nebari.

Three or four years ago the same Ficus has small aerial on either side of the trunk. They are pulled close to the trunk and held in position

Aerials are now fused to the base, increase the taper and thickeness of the lower trunk