Gotta piggy-back on what Lifted said (who by the way gave me some excellent advice on rejuvenate my erection). Dr. Lin actually covers this on his actionlove site if you are willing to search for the post.
He also mentions how the added dimension of the PGE hormones of the vagina help aide in expansion, coupled with the sucking mechanism of a woman's uterine floor during orgasm. I totally dismissed this when you asked it a while ago, but there may be some basis behind this.
Now for the easy part, all you have to do is master ballooning techniques on your own and go get a harem of hot horny women to help you achieve your goals!