Your thickness and prospects for improving size will be based upon increasing your volumetric capacity, the only way to do this is by taking in more blood and improving the pressure gradient of your erectile system. We all have different capacities and limitations to realize this new size. How would you go about making a toilet paper tube bigger? It would be next to impossible without rupturing the paper roll since it has minimal elastic properties. We all have different elastic capacities and the ability to maximize these properties. Improving your cylinder and the vascular pathways to hold more blood is the only way to gain thickness.