The Icon property is template bound to a native Image element in the ControlTemplate of the RadRibbonBackstageItem. This image has MaxWidth and MaxHeight properties set to 16px and this is the reason behind the reported behavior.
In order to alter this you can extract the ControlTemplate of the RadRibbonBackstageItem and slightly modify it. Basically you can find the Image with x:Name set to "Image" and change its MaxWidth and MaxHeight properties. For your convenience I attached a project with the modified style.
Please let me know if you need any further assistance.
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