The size difference is really minimal as I can put the slender half of the 808 in the Nokia leather pouch for N9
(the one that was hardly usable because of fitting too tight)

Also the back of the device has a more round fit in the hand palm while still maintaining a sharp corner on the display side for improved finger grip.
Combined with weight, less slippery plastic (that may well become slippery after extended use) it feels like providing a more solid grip.

For an N9 to have good grip you need it to wear that rubber. That makes it again thicker. (also somewhat uglier and prone to catching dirt between the rubber and the housing you are expecting it to protect)

The 808 you can comfortably use w/o rubber, not risking it to jump out of your hands by the first attempt to snatch a quick shot from your bicycle.
They also provide a cheap wrist cord made in china but I don't like the metal part on it.

Size in pocket:
Some reviews go as far as to complain about the pocket-ability of the device due to the camera bump.
I don't want to know what kind of trousers those NYC mobile blog hipsters wear during their weekend night out or what rare form of elephantiasis they are suffering