Sounds strange. Assuming that this is not simply a viewing artifact (have you tried increasing the viewing zoom?), I'm guessing that you are somehow picking up a style feature at that size. you can try selecting the paragraph and pressing Ctrl+Spacebar, then Ctrl+Q, to clear any "overlays" of character or paragraph styles. If this doesn't help, I'm not sure what you've run into. If you post a page of the document, I could take a look.