I think TI82 is right on this issue. The image become bigger than you expect because its dpi doesn't match the dpi of your system setting.
A simply workaround is to bind the size of the Image to the PixelWidth and PixelHeight properties of the BitmapSource and set the Stretch behavior to Fill. These two properties are calculated against the pixels of the image and the dpi of the system settting. So the Image will be of the exact size you want.
Here is an example:
Image Stretch="Fill" Width="{Binding RelativeSource={RelativeSource Self}, Path=Source.PixelWidth}" Height="{Binding RelativeSource={RelativeSource Self}, Path=Source.PixelHeight}"/>