This appears to have begun shortly after the changes for T226503 were pushed out

On any page, on any project when logged in and using the monobook skin, the horizontal page layout is now very very wide, causing horizontal scroll bars to appear on browsers, and making the user think there is content off the edge of the screen that is missing.

See on test wiki here: https://test.wikipedia.org/wiki/Main_Page?useskin=monobook