Let's face it - these guys are measured, and promoted, in porn inches. Which means that size typically gets inflated by 25-33 percent.
Personally, I reckon the problem dates back to the 70s, before video, when a tight close up of a male star could put an image maybe 15-25ft on a cinema screen - which made it easy to impress. Even today you'll see claims that the big star of the day, John Holmes, was 12 or 14 inches. Actually, taking his known height, it's possible to estimate his true size at somewhere between 8 1/2 and 10 inches. To back that up, Rolling Stone magazine once reported that Holmes's first wife said that early in her marriage she'd come home to find him measuring himself. To her, he claimed to be 10 inches.
IMO the biggest guys in the porn business today are 9-11 inches. Lexington Steele claims to be 11 inches and he probably is. Mandingo (in most of his pictures - not the ones posted on this thread) looks to be about the same length but has quite a bit more girth - in volume terms he has L.S. beaten by some way.
Doubt I've ever seen a real 12 incher - if they exist they are incredibly rare.
What does this tell us? Only that the porn industry's got itself caught in a position where exagerration has become the norm. If the top stars are supposed to be 11, 12 inches or more (and you see Steele and Mandingo boosted as 13 or 14 inches pretty regularly), who's going to want to watch one who's 'only' 8?
'Course, it's not just the porn industry that does this. We all know that most men exaggerate size all the time - to girlfriends, in bed, when they're answering questionnaires, to themselves... hell, on this site. Surveys that allowed men to measure themselves consistently report bigger average and extreme sizes than those that don't - what a surprise. The biggest moderated survey I come across reported the smallest average size - 5 1/2 inches.
I only have my own experience to go on, but I'm "only" 8 1/4 inches (posted some pictures yesterday in the members gallery, so you don't need to take my word for it) and never had a girlfriend who's had bigger. And I've seen plenty of guys on the net who claim to be 9, 10 or 11 inches and are clearly quite a bit smaller than me.
Take all such claims with a large pinch of salt and you won't go too far wrong.