This is because of extensive racial mixing.
Not quite, that's part of it. At one point in time "humans" all had darker skin, the same level of melanin (pigment), but as people started migrating northward they were exposed to lesser levels of vitamin D produced by sunlight, the people with the darkest skin couldn't absorb any vitamin D, and over a very long period of time, that level of melanin, which is genetic, dropped. There is a very large range of melanin levels in Africa, because we spent most of our earlier time migrating up and down that continent. Now, sixual selection, has some influence because a child gets half of their red and black pigmentation from each parent. I would also not use the term racial, because there biologically speaking, exists one race, the human race, all of the other races of Homo Sapiens, such as the neanderthal, died out thousands of years ago, skin tone is just one of the most varied feature in our race, and it's become the prime way for people classify and segregate. Race is purely a political/social term which has no basis in reality, but of course it depends on how you choose to define the term.
Sorry for that, but to get on topic, I like the Red-gaurds, and their historical background. They're technically not part of the human blood man, maybe from a very long time ago, but they make great warriors. I often times don't choose them, because I connect more with the Dunmer and Nordic race. I think the race that I have never played are the Altmer.
I think he was actually meant lithe and athletically built. Not the beefy 'n' stocky types a la the Nords and the Orcs.
Yes, the Red-Guards are supposed to be very fast, and it would make sense for them to be less stocky. Much like they were in Oblivion, of course everyone had the same body type in Oblivion...