According to both Notch's Twitter (no link) and the wiki, maps have some kind of settable zoom:
A zoom level, which will somehow be decided at the time the map is created.
I have no idea how to set this. Anyone know?
EDIT: based on fjdumont's answer, how is it calculated? Is there anything I can do (such as craft it in a certain place) to give it a zoom level close to what I want?