Content not filling container width

Hi,
I seem to have done something, and I’m not sure what, but suddenly the content on all of my pages don’t enlarge with the browser window. Everything, including the header menu, is squeezed into a 900-something-pixel container, so the images are smaller than they should be. What’s weird is that everything was fine earlier and I don’t remember changing anything in the Settings that might be related. Also, I have made sure that I’ve selected “stretched layout” in General Settings and that the maximum container width is 1310px, but it’s still doing it.

Thanks in advance for your help.