To recreate:

1. Create and activate an overworld map, zoom it out to max with 4x8 sheets of paper.
2. Go to the Nether.
3. Create and activate a nether map, zoom it out to max with another 4x8 sheets of paper.

Result:

The area covered is 1/4 that of the overworld map - i.e. one zoom level further. The starting area (and intermediate stages) are also zoomed out by one, but the glitch is unmistakable when you get the the final zoom level.

Similar issues may apply to the End as well but I haven't tried that.