If you are comparing yourself to others and feeling like you do not win the numbers game, if you look at the percentage of increase, you certainly do win. Expanding your size by a multiple of two when erect is way ahead of most men. Most men don't gain four inches when erect. Some whose flaccid size might be bigger than yours do not gain much more than an inch or two when they get an erection.
All you need to care about is if your sexual partner gets satisfaction from your six inches (which is a nice, even slightly big, size). Most ladies' vaginas are no deeper than five or six inches, and despite all the porno images of men with huge penises, most women would find that painful, not desirable.
The one time I had a lover that had a small size when erect, his penis might not have been more than three inches long when erect if that, and it was very small around, and the odd thing was that he was a tall man and had big feet. Old wives' tales would have a tall man have a big penis, but no. My lover with the biggest penis was an average height man. He did not have an unusual length, but his penis was very thick, and that was very uncomfortable to try to accommodate. So as you can see, having a nice, natural six inches is quite enough.
Unfortunately alike to women feeling self confident in their appearance from reading too many glamour magazines, Men also suffer from a similar case, usually from watching too much porn, where all the guys have permanent erections, and along with camera angles, drugs and exercise (Yes, you can exercise your penis) it can make the normal man feel a bit sub-par, but fear not, I wouldnt say youre any different to alot of men, 6 inches is plenty for a woman.
if you really feel the need to try and increase your 'member' then there is a very long process of exercise i believe called the 'Kegel Exercise' try Googling it.