How can I choose the zoom level for my map?

From version 1.4, you can zoom maps in game, as the whole process of map-making is changed:

1. First, a blank map is created
   
   This blank map will not start recording immediately, as maps previously did.

2. The blank map can be activated by right clicking it, which turns it into a map at the most zoomed in level (1 px/1 block)

3. To scale the map by a factor of two, take the map and recraft it as follows:
   Each time this is done the map will scale out, so from 1px -> 1 block, to 1 px -> 4 blocks, to 1 px -> 16 blocks and so on. This can be done a maximum of 5 times. Maps cannot be scaled back in, only out.

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.

Looks like yes, this is the work of a mod. Here's how you can do it:

1. Download NBTedit from here
2. Unzip the ZIP file
3. Run NBTedit
4. Click File -> Open
5. Navigate to your save folder
6. Go to the "Data" folder
7. Open the map_0 file. If its not map_0 you want to edit, choose the right one.
8. Expand the "data: 7 entries" branch
9. Change the "scale" value to the one you want. I chose 1, but it can be anything.