How to reproduce: 
1. Use the following code to change the default style of the document:
StyleDefinition h2 = this.radRichTextBox.Document.Style;
h2.SpanProperties.FontFamily = new System.Windows.Media.FontFamily("Microsoft Sans Serif");
h2.SpanProperties.FontSize = Unit.PointToDip(8.25f);
h2.ParagraphProperties.SpacingAfter = 0;

h2 = radRichTextBox.Document.Style;
h2.SpanProperties.FontFamily = new System.Windows.Media.FontFamily("Microsoft Sans Serif");
h2.SpanProperties.FontSize = Unit.PointToDip(8.25f);
h2.ParagraphProperties.SpacingAfter = 0;

StyleDefinition hyperlinkStyle = this.radRichTextBox.Document.StyleRepository["Hyperlink"];
hyperlinkStyle.SpanProperties.FontFamily = new System.Windows.Media.FontFamily("Microsoft Sans Serif");
hyperlinkStyle.SpanProperties.FontSize = Unit.PointToDip(8.25f);

2. Add some text to the document

3. Add a hyperlink somewhere in the middle of the text

4. Start moving the caret so it moves over the hyperlink start

Observed: When on hyperlink start/end, the caret is with bigger size than the content

Expected: The caret should be with the size of the content