You are correct it does change when you change globally just not very much. Several other phones (HTC One for example) allow you to change the font size globally as well within the stock text app.
That's for sure. I have 2 unlimited lines and for the privilege of upgrading with V I would have to change to share everything and pay $50 more per month. If this is not fleecing customers I don't know what is.
Does anyone know of a way to change the text size in the default test message app? I have the font size under display set to largest but it does not seem to effect the font size in the text app much.