There's another way to look at this. The girlfriend mentioned that the OP was the "biggest" she'd ever had, indicating his size with her hands. It was all "in good humor." Guys sometimes find it hard to believe but most women don't give a shit about the size of a guy's cock. They mainly care about how it is being with him and how the sex is overall. Most the time, that doesn't depend upon the size of the cock. Nor do they care how they get off. I've learned from personal experience, most -- or at least a good percentage -- are just as happy getting off from oral sex as from a cock. Maybe even more. I've had many women blurt out something about the size of other guys' penises, or about mine. That's usually not a problem. But sometimes it can blow up in their faces as some guys are really insecure about the size of their cocks, often for no obvious reason.
I had a vivid example of that once. When I was a year or so out of high school, I went as the "guest" of a friend of mine to his health club. Frank had a huge cock. He once told me it was 9.5 inches; his girlfriend bragged it was 10. He was like a great athlete or rock star at the gym. It often drew lots and lots of attention. Frank ate it all up, grinning like a kid with an ice cream cone. (Frank, after all, wasn't a great athlete or a rock star. . . but he did have a huge cock!)
One day after a work out and a shower, a group of the guys gathered around and peppered him with the usual questions. Like what type of underwear did he wear? Frank, basking in the limelight, answered the usual stuff about the problems with different types, adding a comment on the lines of "you guys don't have to worry about those type things. . . " when Tom Z, one of the guys I'd known for many years, a fairly good-looking "hillbilly" type with a decent build, all of a sudden burst out screaming things like "Keep your damn mouth shut! Keep your filthy mouth shut!" and the like. He really lost it, totally ballistic. We were all stunned and Frank was instantly apologetic, "Hey, man I didn't mean anything. . . ", Frank hadn't a mean bone in his body. But he'd somehow really struck a nerve with Tom Z. And for no reason that we could see. I'd seen him many times in the showers. He had a thoroughly average-size cock, what 50% or so of guys had.
You never know. Your friend may have been like the guy at the gym. His girlfriend obviously thought his cock was just fine and never guess how insecure he was about his size.