The races in The Elder Scrolls series haven't changed much over the last few games. The history and lore of the series is highly detailed with countless layers of depth. The only major difference in any of the games was that Morrowind had beast races with extra joints in their legs. It was an experiment to make the more complicated legs. In Oblivion they decided not to mess with it. If they tried to put the same legs into the game again, it would have been released late. Not only that but all Kajiit and Argonians would have to go without boots again.
The lore mentions that there are many different kinds of Kajiit. Some look more human and some even look more like cats/lions and walk on all fours... but they mostly live in their homeland (not in the game).
As for you other questions... It's a pretty complex set of conditions that affect the races, just like here on Earth. It's cultural. The Nords live in the frigid North. They've live there for countless generations and they have to be strong to survive up there. They're naturally bigger and favor strength as an important trait because of their environment. There's a lot of information out there about each race in the game, the in-game books, and a lot of it is collected online.
The Unofficial Elder Scrolls Pages has a good page on
Races. The first page is just game stats but click the race names and they'll give you a little more info.
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