I guess you could used the Align property of the RadPaneGroup to arrange the tabs, when setting it to Justify, they should cover the whole area. The default is Left. Also, make sure that any header content templates that you might be setting are sized properly, and there are not Width setters to your RadPanes.
If these do not help, could I ask you to give more details about your use case, specifics of the project or customizations to our controls and if possible a sample project isolating the issue, so we could be of assistance.
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