When the comments of the document are shown (RadRichTextBox.ShowComments="True"), the document has to be scaled in order to have the comments show in the printed document. As the height/width ratio must be kept, this causes shrinking of the text in height, too. You can see that this is also the behavior in MS Word when there are comments in the document and they are shown at the time you print the document.
You can hide the comments in order to print just the body of the document and then there will be no additional white space at the end of each page. This can be done using the ribbon UI - Review Tab -> Show/Hide Comments.
I hope this helps.
the Telerik team