Why does the font size change from Notes to Mail

When I email a note to myself from within the Notes app on my iPhone, and then look at the just-mailed note in the iOS Mail app, I see that the font size has increased greatly. And when I view the emailed note in Apple Mail on my Mac, the font is huge. But if in Notes, I just do a normal copy, then paste that into a new email in the Mail app, the font doesn't change and looks just right, in the Mail app and Apple Mail on the Mac. Anybody have an ideas about what's going on?

Ken