What do you see when you enter preview mode?
Neither saving as PDF nor printing should change the size of your text from what you have on the layout.
When I switch to preview mode the text appears as normal -- most of it is font size 12
Edit: do you think it could be some kind of dpi issue?
Perhaps. If it was just the printer, I'd check to see if the printer is substituting a font, but since it's also happening on a PDF file, I'm not sure what would cause that. The Preview is supposed to show you what you'll get when you print or pdf the layout.
Try creating a test layout (don't copy anything from the original layout) and see if it shows the same behavior.