hi,
check the "regions" folder of your map. you see files like r.-1.-1.mca and r.-1.-2.mca etc.
that's the chunks near your spawnpoint and usually the ones that you use and build at.
your map has chunk files like r.1952.1952.mca, that chunks are far, far, far (far far far) away from your spawn. you need to remove them (make a backup copy!!) to get better zoom levels on your spawn area.
usually, this chunks are empty and were created by nether portals. i'm not sure if yours are empty, as the file size is rather large, so don't forget the backup copy..