If you found your text-messages/SMS to be mysteriously huge in font-size (as shown below) for no reason, without changing anything in your settings, the reason is your phone using the "VOLUME ROCKER" to change the font-size of phone's native texting application. To bring back the text font-size to desirable setting again, start the messaging app and use the physical up/down volume control button of your phone to get the desired size.