By default, OmniGraffle 7 for Mac will show previews of each Canvas in a document. It’s useful to get an instant visual on the Canvas’s contents, but some prefer to fit as much data on the screen as possible for larger documents.

To switch over to small icons to represent your canvas, Control-click anywhere in the Sidebar and select “Use Small Icons” from the shortcut menu.

The result will put quite a few more Canvases in view.