Between British and American ladies

Those mean different things in American and English

Americans for sure, and not (just) for the stereotype of them being bigger overall. The meaning of fanny plays a big role too.

Whoever opens their legs the widest

Those with genetic predisposition to big fannies. Obviously.

Vaginas or asses? Fanny means different things in each country

Fanny is an old slang term for the female butt in the US. It was going out of style even when I was a kid. I would be surprised if your typical Gen Alpha had heard of it.