I think people underestimate the difference in APPEARANCE of cock size only fractions of an inch make. I mean the difference between average and very big is probably only an inch, so a 7.5" cock is going to look huge next to a 6" cock.
Also, people underestimate the optical illusions created by things such as cock girth (which makes a cock look much shorter or longer). For example Criss Strokes looks way longer than Manuel Ferrara but in reality they are within an inch of each other. The size and shape of the MAN's body makes a big difference too. Take Biggz for example. He is probably 5'6" tall, 115 pounds and has a relatively skinny dick and he is usually paired with extremely petite girls, so his cock looks 15" long, when in reality it is probably more like 8.5" (which is still among the longest).
For all those reasons, I think most people OVERESTIMATE Danny D's length. He has a relatively thin cock and is very skinny. I think he is probably 8". Just my two cents.