Zoom property internally is coerced to a valid value. A valid value mean that it should be greater than zero and not infinity or NaN. In addition, RadDiagram performs a secondary coerce in order to support custom business scenarios (for example you could probably would like to restrict the users to zoom more than four times). The second coerce trims the value between DiagramConstants.MinimumZoom and DiagramConstants.MaximumZoom values.
Having that said, you could set the DiagramConstants.MinimumZoom to a really small (but greater than zero value). A value of 0.001 for DiagramConstants.MinimumZoom means that you will be able to zoom out to a factor of thousand.
Hope this helps. Please let us know if you need more info on the topic.
Furthermore, it looks like you are using RadDiagram in quite complex scenario. We will be very glad if you share your comments (expectations of how the zoom out should work) in order to improve the zooming behavior?
the Telerik team