Just reposting this because I edited my post without seeing your new ones.
Basically any adjustments whatsoever don't adjust my configurations, so I believe it's on Blizzard's end.
I disabled all addons, and when I tried to go from 1900x1080 resolution down to 760x576 I got absolutely NO changes. Here's my screenshots:
With nUI, my menu is open for comparison of the drastic size. (I've never seen it that small)
Here's without nUI, at my maximum settings, the ones I usually play at. Note you may not be able to tell, but they're still small for me:
Now here it is with the lowest settings available to me:
