In an effort to optimize the spectroscopic performance of large volume Cadmium Zinc Telluride (CZT) radiation detectors grown by the Traveling Heater Method (THM), especially for higher energy gamma detection applications, bulk material and pixellated detector performance of THM CZT crystals at 15mm-thick and 10mm-thick were compared to that at 5mm-thick. We demonstrate that the outstanding performance of monolithic pixellated THM CZT detectors was thickness-scalable. For the same monolithic 20×20 mm2 “Compton Camera” pixellated detector configuration, detector performance can certainly be significantly improved as the thickness of the detector increases from 5mm to 10mm and thicker as demonstrated via both 57Co 122 keV and 137Cs 662 keV spectral responses. Depth of interaction information obtained from 3D depth sensing measurement system, IR microscopy and x-ray topography were used to study the correlation between structural quality and detector performance. Additional material characterizations and detector performances of various sizes and configurations were also presented and discussed.