To reproduce this:

• Set the RadRibbonButton LargeImage/SmallImage to a property that is larger than 16 (small image) or 32 (large image).
• Set IsAutoSize to False. This will stretch the image on all available space for the button.
• Set IsAutoSize to True. This should uniform stretch the picture so that it fills the auto size of 16 or 32 pixels. However, the image doesn't stretch, instead its Stretch is set to None, which expand the image to its original size. This leads to the clipping.

To work this around, avoid setting the IsAutoSize property. Or use smaller pictures - 16 or 32 pixels.