Sorry folks, I tried to find an answer to this by searching for stuff on map scaling/resizing, but I'm not sure the answers I've found are the same thing I'm experiencing.
At one time, many versions ago, I create several maps using a campus drawing to show physical locations. I used a big background for clarity (3700x2900 pix). When I originally created these maps, they were understandably huge. I seem to recall, at the time, I found some feature in the map editor (this was prior to Atlas) that allowed me to scale down the map while I was editing it, but I didn't like the results when viewing it from the web console so I left it at 100%.
I'm not sure when something changed, but at some point along the way of the NPM getting updated, and the arrival of Atlas, and more software updates, etc..; my big campus maps were auto-scaled/resized down - similar to what other have reported only I'm not showing these map on a customized column page, I'm just launching the maps directly (going to a List of all Maps and selecting a campus map) which takes of the whole screen.
As a test, I just created a new map using another (updated) version of a campus layout background (different file name) with the same dimensions, added some objects to it, saved it with a name that was unrelated to previous campus maps, went to my list of maps, and once again it was scaled down and unreadable.
How do I stop it from doing that?