The RadDiagramShapes doesn't listen for changes in the properties of their Geometries. This is why the RadiusX and RadiusY properties of your EllipseGeometry are not updated when you resize the shape.
In order to update those properties you can subscribe for the SizeChanged event of the shape and modify them based on the shape's width and height in the event handler.
sender, SizeChangedEventArgs e)
RadDiagramShape shape = sender
var geometry = shape.Geometry
geometry.RadiusX = shape.ActualWidth / 2;
geometry.RadiusY = shape.ActualHeight / 2;
Please let me know if this works for you.
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