How can I change the function of the volume key in messenges? I have a Galaxy S5, android version 5.0

When I have the default messenges app open, if I press the volume key it increases or decreases the size of the font rather than the volume. Is there a way to change the function of the volume keys
Open the message app. Press the menu key. Choose settings. Choose "Display". Uncheck the option change font size.