Exported PDF has enormous canvas size, with my design in the corner
 

Something mysterious has happened with my wireframe document mid-project. Now, when I export as PDF, the pages are enormous and mostly blank, with the canvas contents in the bottom-left corner. But they used to export just fine (I use this template for all my projects) and I haven't made any changes relating to canvas size or page setup. What happened?

The templates I use for wireframing use a custom Paper Size of 55.88 x 86.36cm, as set in Page Setup – this applies to the whole document, right? The Canvas Size inspector is set to 1 page x 1 page, Size multiple, Auto-adjust. All settings are identical to other documents which don't have this problem.

If I export a single canvas, then the resulting PDF has the correct page size. The moment I export more than one canvas in the document (any canvases), the PDF page size is enormous.

Can anyone help?