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?
You can zoom map files, but you have to do it by editing your NBT files. This answer is thanks to this redditor:
If you’re willing to dive deep into the
NBT files, you can adjust the map’s
center and zoom levels. Just don’t
screw with dimension (locks the map)
or width/height — it doesn’t work. The
files are under
<map-name>/Data/map_X.dat, where X is
the map number.
You’ll need to use an editor like NBTEdit to edit the files.