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This is really just of my own curiousity.

What exactly causes a bike to start under much more effort when it's cold? If it's 40 degrees out, I have to fuss with the choke and ride the throttle and crank and crank and crank.

If it's 70 out, it starts in about quarter of a crank.

Is the mixture less volatile under colder conditions? Inquiring minds want to know.

Yes, gas does not vaporize as well when it's cold, which makes it harder to ignite, and not mix with the air real well.
I'll bet that wiredgeorge could help you make it start a lot easier though, by making changes to the carbs.

not only that but cold thick oil, cold spark plugs contribute to it......