johnny9 is right: it is different for everyone. I love to clamp, but even clamped as tightly as possible (last click on a cable clamp) I get virtually no girth expansion beyond my normal EG. The glans expands, although more accurately it simply stays at max expansion, but nothing more than a hard Kegel at full erection gives me.
For these reasons, I don’t think clamping is going to be a source of growth for me. Heck, it might even be impossible for me to gain girth, although someday I will try pumping and see if it is any different.