What's the big deal with 1.75mm and correspondingly smaller extrusion nozzles? Here's some things to consider:

• Thinner filament can be heated faster because it takes less time for heat to penetrate to the centre of the filament, implying faster printing and less hefty heating elements
• 1.75mm filament weighs less per unit length and thus the extruder mechanism has less mass to worry about when shifting to and fro
• Thinner filament can be bent more easily, implying smaller tighter and smaller coils and possibly smaller storage mechanisms
• Smaller nozzles are more capable of avoiding ooze when the flow can be slightly reversed by the extruder's stepper motors

from http://fabbaloo.com/blog/2011/11/21/the-end-of-3mm-filament.html