So I dove into the help system, and the first hit was the right one.

A topic called, of all things, “The text of my message is larger or smaller than usual”:

If you read that closely, you realize that what we have here is a really bad case of product design and bug fixing. Essentially, they have put in a documentation fix for a bad UI problem. This text is a plain admission that people do this by mistake and have no idea what they did to cause it, or even that they are seeing a zoomed view. The root problem is that the zoom activitity can be done by mistake, by the shortcut “ctrl-scrollwheel-on-mouse”. Microsoft must have noticed this in testing, and put in a piece of help to help you out. But before you can find that, you need to figure out that you should search the help for the issue.

If you do not have a scroll wheel or don’t know this, the solution is to get to the format text tab, click zoom, and then change settings in a dialog box. Amazingly bad, consdering the effort going into Outlook:

The correct solution is NOT to document it like this, but rather to just have the same little zoom control as all other Office programs and remove the easy-to-hit-by-accident shortcut (maybe it can remain if the effect is clearly seen in the zoom control).