Antenna effectiveness depends almost entirely on wavelength, frequency, and amplitude as well as the antenna shape - not necessarily size. Ie, a smaller antenna, such as 1/4th that of the actually wavelength, would work much better than that of one that was larger and 3/4 that of the actual wavelength.
Have you noticed that on luxury cars the antenna length changes based on the frequency you are tuned into?
Not to get into an argument that Im hardly qualified for, but bigger isn't necessarily better the way I understand it from college
All that to say - Im hardly educated on what Im talking about (it's obvious isn't it) but I think at least one of the points of putting it on the outside was to make it external vs internal, thus less interference... THAT and to call it a "feature" ...hah!
http://en.wikipedia.org/wiki/Antenna_(radio)