Have you taken a look at the Canvas: Size inspector - you are probably using the Printer margins option (all printers have margin settings by default - mainly because they can't print right up to the edge of the page):
If you pick your own settings for the printer margins (just make sure you use values that mean the image will not extend into the region of the paper your printer can't actually print to) you can use equal values for all edges and the space will be the same around each ticket.