I've read a lot of reports on the forums through searching in google. A lot of heavy girth gainers have gained 1 - 1 1/4in girth and attribute it mostly to jelqing combined with horse squeezes and Ulis.
My theory is by jelqing your training your cock to supply more blood flow in a moving fashion up the shaft. That's why after pumping it doesn't stay forever. Bc it's not accustomed to holding more than it's normal amount of blood. I'm sure everyone knows this.