Same here. I have 2 theories just to be realistic. 1. We see more white penises than black cocks 2. A black guy with a cock that he feels does not measure up to the image isn't going to get naked. Where white guys do not have the expectation from the image and see other small to average white guys naked so there is no pressure.
Nothing is an absolute but from my experiences black cocks do tend to be bigger. When we sought one out for sex, we looked for a guy who met the larger size. Some of them showed up promising 9" and delivering around 7". I had an ex who at first would fuck them anyhow because it was still much bigger than mine. Plus she felt like if she got them hard she owed them release. After a few that she fucked even after they misrepresented what they had, she just went to sucking them off or a handjob.
As far as flaccid from what I see - and I go the gym very regularly- the guys that show their black cocks are noticeably bigger than most (but not all) white penises. Just my 2 cents and trying to be realistic in that we can't say all people of any kind are the same on anything, everyone is unique.