The bars can have indentations to shorten the reach to the levers. The bar may also be shallower front to back and top and bottom. I don't think the overall diameter itself is smaller but critical spots are manipulated to decrease the reach. The diameter would need to remain the same as other bars so that your components will clamp properly.

Here is a sample.
http://www.terrybicycles.com/detail....&item_no=22000

TTT is also known for making a women's bar. Salsa used to. It was called the Poco (I think ).