Spray width is controlled by a number of factors, including thickness of the meduim being sprayed, air pressure and distance from the subject being painted.
However, if you are drawing the trigger as far back as you can and you can't get a pattern wider than 1 cm, it sounds more like an airbrush issue.
When you draw the trigger as far back as you can, how far down the trigger slot does it go?
Referring to the parts diagram here: http://www.badgerairbrush.com/150parts.htm, if the Tube Shank (Part# 50-030) is screwed too far into the airbrush body, it can limit the trigger travel.
If this is the case, you need to unscrew (counter-clockwise) the assembly to retract it a little from the trigger assembly. Note that the needle is locked to this assembly, so when you do this, you will need to loosen the needle lock nut (#51-010), re-seat the needle and retighten the lock nut.