Let's face it, our cocks are made up of primarily erectile tissues, ligaments, vasculature, and blood.
With the procedure, you are adding a new mostly collagen based tissue directly on top of the erectile tissue. Your cock gets hard, the collagen flattens out against the erectile tissue, and looks mostly normal. I.e,. another ugly fat cock.
This is why the Dr. uses more or less CC's of fluid for every patient. Your erect oem length and girth size is his only guide during the first procedure to determine how much to add.
A naturally thinner cock can obviously add less girth via the procedure than a naturally girthier one, - at least without seeing unwanted side effects. Like bunching, creasing, etc.
At 4 7/8" OEM, I took the two shaft procedures to the limit. I had very strong collagen growth, and at 7.5" today, if I don't have an excellent erection, or if the pussy is especially tight, I will inevitably see some bunching and creasing. Cock rings are not an option for me anymore as they trap to much fluid, and make the shaft spongy, not firm.
Hope this helps, 7.5" is a monster cock, and my wife loves the overall utility it provides to her in having massive orgasms, but it simply doesn't look like a natural cock anymore.