Hello Jeff,
As the transition is between a screenshot of the old state and the new state itself, they should be of the same size. In other case the image is either stretched or shrunk. The TransitionControl is designed to work this way.
In order to overcome this limitation you may use the FluidResizePresenter control located in the Telerik.Windows.Controls.TransitionControl namespace of the Telerik.Windows.Controls assembly - it makes a fluid change of the size which will make the transition between different sizes to look better. What you need to do is to just wrap the RadTransitionControl with it like in the following example:
If you have further questions don't hesitate to ask!
Greetings,
Miroslav Nedyalkov
the Telerik team
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