All the settings pages seem to be half width, both the core pages such as maintenance mode setting, and pages in my own plugins. Its the same in firefox and chrome. I never noticed it before but I dont think its been there for a long time, its just that my settings pages are now really cramped when displayed left and right spans.
This is indeed a new feature where the form size is artificially limited to offer better usability. Since not all forms need to span the entire width, it can be unwieldy on larger displays.
We’ve added an option to specify the size in the next version (v3.1.4) along with the other properties.
Property
Description
size
the settings form size when used with a class definition. Supported values: tiny, small, medium, large, huge, giant, adaptive. Default: large.
Setting the size to adaptive should restore the previous fluid width. For example: