On the commentary for GF1,Francis mentioned he didn't want to include certain aspects of the
novel in the movie,specifically a subplot involving a young girl who needed an operation
because "her private parts were too big".I've never read the book so I'm unfamilar with this.
A book about the mafia had a subplot about an oversized vagina?Could someone explain the full
extent of this subplot.Thanx in advance

The girl in question was Lucy Mancini, Sonny's paramour at Connie's wedding, and later his mistress when he and his guys are "on the matresses" during the Five Families War of 1946. The novel has a subplot involving Lucy and her lover, Dr. Jules Segal, physician at the hotel that Moe Green owns and where Freddy Corleone is in training (and getting slapped around). Jules advises Lucy to get her insides tightened, having had his attention drawn to her condition after making love to her. And he's present at her operation. Jules also arranges for one of his surgeon pals to fix Michael's face.
This was a fairly interesting subplot in the novel, but in my opinion, Puzo included it so he could go into detail about Lucy's operation, repeating some medical facts that he'd picked up somewhere (just as he went on and on about Hollywood practices in "The Last Don." I think FFC was right not to include the Lucy/Jules/Vegas/operation subplot in the film.

The plot would have fleshed out her character for the introduction of Vinny in GF3, so i believe it would have been merited for some inclusion

Yes, but The Godfather Part III was an afterthought.

i always thought her "tightening" subplot as a strange and unnecessary one unless we needed further evidence of the incredible size of Sonny's manhood . i am glad FFC decided to leave it out as he did with most of the Lucy story. Lucy was also infertile in the book, so Vito was a big afterthought for GFIII.

in most aspects...but some people dont look at it in that way.

The chick with the big pussy was Lucy Mancini, the chick Sonny was banging aagainst the door in GF1.