Normal road bike rear spacing is 130 while MTB's are 135mm. The different width hubs are designed for different frames (road/mtn/track/etc) and they are specifically designed to run a better chainline. For example, the Surly New SS hub is for 135mm spacing and is designed to run a 52mm chainline which is what works best for a MTB. Hopefully that makes some sense. To answer your original question, the hub itself is normally bigger but it can depend on the manufacturer. Check out http://www.sheldonbrown.com/harris/fixed-hubs.html for some additional info. This may help or it may just confuse you a little more. You could always talk to the guys at your LBS for some additional and visual help. What kind of bike are you gonna try this one?