RadButton (ButtonType="StandardButton") uses for its background sprite images with fixed height of 22px (default height) and 65px. From the provided code snippet it seems that your RadButton is set with a custom height of 35px. If you want, however, to use a custom height you should create your own sprite as well as modify the CSS of the RadButton. This help article
will spread more light on how to do that.
You can also consider the new RadButton (ButtonType="SkinnedButton") that uses entirely CSS3, so that you can set a custom width and height to it.
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