I say it has nothing to do with studio lights being hot. Most studios are kept cool from what I've heard.
It's about looking good, hot young you name it.
One of our local channels has a weather guy that is downright chubby if not larger. He doesn't have to do anything about it because he's a man. Men can be homely, large, old, whatever and still get the job and keep the job. Woman have to stay fresh, young, flirty.
It's an old stereotype that needs to go away.
I loathe seeing women on the air on any tv channel going sleeveless in the winter especially when they are talking about the horrible cold weather just outside their window!