I have the same issue; my girth is maximum (a hair above 6'') between one-third and one-half of the way down the shaft (starting from the head) and eventually tapers down to noticeably less near the base (never measured it there, but could be around 5.5'' or thereabouts). I have noticed though, if I get a really strong and sustained erection (which is unfortunately rare these days) it seems as if just a little bit more of the inner penis gets pushed out, and my (new?) base looks noticeably thicker when that happens.
This is sort of tangential from the main topic, but I'm wondering if there's a relation between masturbation style, and the shape (and possibly girth) of the penis? The way I always did it was with slow, hard, compressing jelq-like motion (with the significant difference that this was a "backward jelq", i.e. from head to base) where my grip tightened the most at the point where my girth is the thickest (sorry for being so graphic, but it's for the sake of advancing Penis Enlargement science) - not the sort of thing that could be done without vaseline. Rapid rubbing with a soft touch [that you often see in pornos when the guy jerks off on the girl] was really never my style.
Incidentally my (maximum) girth of 6'' or so seems to be above the average, if i'm not mistaken, because a lot of people state their goal size to be 8'' x 6''. If that's the case, then I wonder if that could be attributed to masturbation style too?
Perhaps some of the other guys who are similarly shaped might chime in and confirm or refute my idea?