In answer to the original question, I would bet the top played races are High Elf (for DPS), Argonian (for healers), and Imperial or Nord (for tanks). BiS has a large influence on race choice.
I would be more interested if some races are populated in particular zones. Maybe it just me, but I see a lot of Morrowind players picking argonians. Well since Morrowind feels like there's a lot more.
I'm a Breton. My mom is a Breton. My dad is a Breton. My sister is a Breton. My aunts and uncles and grandparents are Bretons. My girlfriend is a Breton. Most of my friends are Bretons.
There ya go. Most of the world is Breton.
Not all, of course. My great great grandfather on my mother's side was a Nord who married a cute little wood elf. And on my father's side, there was an Imperial who had a torrid affair with an Altmer, or so I've been told... when you're a Breton, the family tree tends to have a few tangled branches here and there...