The pixel density of the Retina iPads and iPhone is 2 times what it is for Desktops and other iOS devices. This can cause the signature pad on these devices to be half the normal size. You can easily work around this by adjusting the baseLineSignatureWidth and height setting in the Signature Pad web viewer calc.
You can use Get(ScreenScaleFactor) > 1 to determine if the device is a retina display. That way you can write a calculation that will work for both kinds of displays.