I think the reason why people wear heavy clothes even when its isn't cold is because of the season. We are in autumn, and people will wear coats unless it's 30 C outside.
All these things about body temperature, I don't think it plays a big role.
And the second important factor is, as was said, what you are used to.
Interestingly enough, there are exceptions to this: I have a Bulgarian friend (she's from Sofia), who has lived here in BA since at least 5 years ago, and she says that she suffers the cold here terribly. Could you believe it? Sofia has averages in January of 2C/ -5C, whereas BA in July 15C / 7C. She just says that here there's a lot of humidity and so she feels the cold much more.
I personally love the cold. And, Hache, there are times where you would need to wear a scarf in BA. I know, very few (usually in a windy winter moring), but there are.
I can't stand this city's heat, and when it's cold it's so relaxing! I hope this winter is like 2007!
Cheers,