I saw a cool study on that. Men with a large difference between their testosterone and oestrogen were into voluptuous girls. Men with a small difference like skinny girls. And it's perfectly proportional. Fun thing is that it works in the reverse for woman. So masculine/feminine tastes match perfectly. Might be some way nature has figured out to make sure that testosterone vs oestrogen levels in the off-spring don't reach extreme levels in any direction.
An athletic body has all the markers signalling health. So everybody likes an athletic body. I think that's just genetically hard-coded.
Nah, it's largely cultural because culture heavily influences what counts as healthy.
Normally when we talk about an athletic woman, we're talking about a bit of extra muscle (especially lower body) with good muscle tone. Just a few decades ago, that would have been considered extremely unattractive. All those old European paintings show ideal women as being around 14 years old and certianly fatter than what would be considered ideal now. Inuit almost universally prefer fat women. Not voluptuous, fat.
And plenty of women find large muscles on men to be unappealing, so "athletic" in the sense of sexual desirability can mean different things from woman to woman.