So what you really think they're the same size?
Depends on the style of porn. If they're doing something like "Blacks on Blonds" they get a few guys with larger than average dicks and find the smallest girls they can to make the size difference appear that much larger. Same thing with the cuckolding themed videos.
If you look for any sort of big cock themed videos without an interracial theme, you find the white guys are just as big. Long before the explosion of interracial videos, it was white guys like John Holmes and Ron Jeremy that were doing the big dick films where you got to see them try to stuff their cocks into cunts that couldn't take all of their dick in.
If you want a more balanced representation of cocks regardless of color, look at the gay porn section and you will find cocks of every color in every size combination there is. You will see big black cocks, little black cocks, big white one, little white one, big Latino, etc. You'll even find the occasional Asian with a really large dick.
Heck, just look at the different photo threads on this site and you will see than not all of the black dicks are particularly huge. However, as most are stills coming from professional porn videos, the bulk are going to be large.
Other threads are dedicated to large white dicks fucking black women.
The thing is that guys with really large cocks are not the norm regardless of color and it actually does a disservice to the average black guy.
On one hand it may help him land a white woman that is assuming he is hung like a horse, then on the other he has to deal with her disappointment that he isn't.
On second thought, unless she is smart enough to "check out the goods" before going to bed with him, there is no real down side for him. He still gets laid and enjoys the pussy whether she's happy with his size or not!
It's the average to larger dicked white guys that get screwed because of the hype with all the women assuming that if he's white, he's hung like a hamster.