I don't know if the inner penis is thicker or not, but I don't think base girth growth is the result of pulling out inner penis.
I gained about an inch of base girth for a length of around the first 1 1/2" when I started using a slider
extender at higher tensions than possible with spring
extenders and later hanging, but in that time overall length gains were less than 1/4", so I don't see how pulling out inner penis could be the reason.