Where are you pasting from? I know that sometimes pasting from Word or other sources keeps hidden tags on the copied text to paste. Causes me big headaches in the day job.
I typed the first paragraph, hit the preview button, selected and copied the text from the preview, then pasted it in the editor window. Same thing happens if I copy and paste from another post.
Here's another fun one. The following were typed as two paragraphs with the same text format, then I went to the beginning of paragraph 2 and hit the backspace to merge them into a single paragraph. The second paragraph magically becomes smaller. When I hit the preview button, the text grows to normal size, but gains the size tags that do nothing (probably because the size tag doesn't take percents).
Paragraph 1.
[size=78%]Paragraph 2.[/size]