As the RenderTransform property doesn't affect the layout it is normal to observe this behavior - the window is not going larger when its content is scaled with a RenderTransform. What I would suggest you is either to set RenderTransform to the whole window or to put a LayoutTransformControl and move the transformation to its LayoutTransform property - this way the transformation will affect also the layout. The first one will scale the whole window and will make its header, borders and buttons bigger and second one will only make its content area bigger and will make enough space for your scaled content.
Hope this helps!
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