Because it won't just be font size. It will be some impractical idea that you've already ruled out in three weeks ago in meetings with your manager, but now you have to drop everything and waste more time spinning wheels to either attempt the director's dumb idea, or you have to spend time explaining to him why you've already ruled it out, and do so without seeming lazy or insubordinate.

While you're doing this special task for the director, your workload is piling up and your manager thinks you're slacking because the director emails you directly and never bothers to CC your supervisor. You can try to diligently CC your supervisor on all replies, and even mention your supervisor in emails (the classic "looping in x"), but rest assured, the director will never use reply-all.