A regions area/viewport is determined several ways, depending on what content is added.
A region can have a simple marker location with no bounding box, so the map will just center on that point at a hardcoded zoom level.
If more than 2 trails are added to a region with geo info we then generate an automatic bounding box that covers the area of the trails. Each new trail that is added adjusts this bounding box. The map now zooms to include this bounding box as the viewport. You can actually see what the auto area is when you edit a region, its the light blue transparent rectangle.
Sometimes this auto-box & auto-zoom are not perfect, so the area for a region can be customized/overridden. One can draw a polygon outline, now the map will use this polygon to center the map rather than the auto-generated box. You can also specify a manual zoom level override.
The polygon can be edited when you edit a region, but the auto bounding box cannot. The marker location of a region also changes to be in the center of the bounding box, unless its manually set.