MP4-20 photos:
http://www.f1racing.net/en/gallery.php?catID=1596
At a quick glance, the front nose looks much wider than the 19/19B and the body work seems even tighter. Look at how little space there is between the driver's head and the outer edges of the cockpit. Side pods look a bit more shapely, too. But I'm really not too sure.
The ultra tight body was what caused a lot of the engine failures during the beginning of the 04 season for McLaren. The engine simply built up a lot of heat inside the covers. I hope they don't have the same problem this year.