I could reproduce this on an iPad Air 2 running iOS 8.3 using preview-over-wifi - no need to export. I am certain this is a bug in Safari. It works perfectly in Chrome for Android, but the combination of fullscreen mode "off" and high-DPI mode on makes Safari size the canvas too large. It seems Safari tries to apply some special-case sizing logic, which simply breaks it. I literally wrote code which was working like this:
set size to width + 1, height + 1: appears correctly but off-by-one
set size to width - 1, height - 1: appears correctly but off-by-one
set size to width, height: appears double size
set size to width * 2, height * 2: appears double size, the same as the "width, height" size!
Since it appears to be impossible to set the correct canvas size, I've implemented a workaround for the next build: it turns off high-DPI mode on iOS if fullscreen mode is "off". You can do this yourself as a workaround - set high DPI off in project properties and it works correctly. Letterbox scale is not affected so one workaround is to use letterbox scale and display it in an iframe (which I'd recommend anyway - fullscreen mode "off" was invented before mobile devices and does not really work nicely for them).