Thank you for your interest in RadRichTextBox!
The most probable cause for such a strange behavior is because of the PrintScaling settings. The default printing which is executed from the RadRichTextBoxRibbonUI sets different PrintScaling mode than yours.
In the given code you define PrintScaling like this:
ps.PrintScaling = Telerik.Windows.Documents.UI.PrintScaling.None;
Could you try to replace the above code with the following one:
ps.PrintScaling = Telerik.Windows.Documents.UI.PrintScaling.ShrinkToPageSize;
In case this does not help please provide further details on your setup. I would be glad to provide any assistance.