Gordo

Gordo
 

I know this means fat, but what's the underlying cultural significance here? Would a woman use this as a term of affection for a male friend or is it reserved exclusively for a boyfriend or romantic partner?

It depends on the region and the woman, but it's not necessarily used for a romantic partner.

Yes, in Spain you only can use it to refer to your partner.
otherwise it sounds weird. "gordo" is like "cariño", "nene", "amor"
so, don't call that to your friends!

It depends on the region. Never in Castille. I thought it was more of a southamerican use.