We (as a society) obsess over length, but that gives a very incomplete picture of cock size. While cocks come in all shapes and sizes, the bottom line is that for proportionally sized cocks (say 8" long by 6" circumference - a circumference equal to 75% of length), small differences add up quickly. The key is not just length, but volume.
So let's stop talking about it's the taboo or the skin contrast - and look at the math. Most of the science tells us the average cock size is 5.5", with black men being about 1/2 larger - so 6" or so. A 5" white guy is only slightly small than average (well within 1 standard deviation), and a 6.5" black guy is only slightly larger than average. So the difference in length is 1", which is less than 20% bigger. Not nothing, but certainly not worth all the hype. But in terms of the displacement inside a woman's vagina, the 6" cock is more than double that of the equally proportioned 5" cock (just over 10 cubic inches versus just under 5 cubic inches). And it goes downhill quickly from there. Suppose the cuck is 4.5", and the bull is 7.5". The bull is not quite double the husband in length, but is almost 5 times the total size. It doesn't matter how good a lover the smaller cocked guy is, the harsh reality is that he just can't compete.