The girth of a plant is derived from the vascular (lateral) meristem. Meristimatic tissue are regions of rapid cellular division, i.e. growth. The height of the plant is derived from the terminal meristem (buds), or likewise the length of a branch is controlled in this same manner. All plant cells start basically the same, and then differentiate as time passes. So, after the bud extends, cells begin to differentiate, this is when the vascular meristem forms. Important to note that it is after growth begins, therefore strong lateral growth occurs much later than initial growth. By removing the terminal bud, the tree must divert its energy into axillary buds (dormant), then they must begin to create new vascular tissue. This slows the plant down, and doesn't allow much time for the vascular meristem to develop and grow. So, if you want girth, it is just a discreet phone call away.....errr........I mean.....
leave the big terminal buds to grow
-Dave