I don’t know where you got the idea that most women can take 9 inches, but it’s not so. There may some who can take the entire length, and of those, a few who enjoy it. This is a very small minority, about as small as the minority of people with dicks naturally as big as yours (less than 1 in 10,000). So if you want to make it bigger for her, you’re doing it for an extremely small percentage of women.
Also, just to clarify, what is your method of measuring yourself?
And yes, you can certainly gain more, you have as much capacity to do so as anyone else, probably more.