I'm having trouble printing a map with a desired scale. I set the scale to 1:100,000. The page size is 30x30 cm, therefore the real world map extent should be 30x30 km. However, as one can see with the scalebar, my real world extent is about 20x20 km. The scalebar is correct, I also checked the map extent on some web maps, the extent really is too small. Since the paper size (30x30cm) is also correct, something must be going wrong with the scale.
I'm using qgis 2.16.3 on Mac os X 10.9.5. Projection of the project is EPSG:3857 (on the fly enabled), projection of the layer is WGS84. It's OSM data from geofabrik.