Page width reverting in the editor

This may not be a responsive question, but it is about layout and the editor.

My pages are set to 1600 width. They keep reverting to 1200 width when I preview them, and when I go back to the editor, they are back at 1200 there.

(I’m not actually designing responsive right now and I’m sure I have a setup error on my end. I just want my pages to stay at 1600 so I can get them built!)

Any help is appreciated!!