Plain text docs respect font size set in preferences for full screen mode but rich text documents do not.
1 - In preferences -> editing, set font size for full screen mode to 18 pt and text size for both plain text and rich text to 12.
2 - create a plain text document - type and notice font size 12
3 - open in full screen mode - notice font size is bigger.
4 - convert document to rich text
5 - open in full screen mode - peer in despair at tiny font / reach for glasses or simply notice that font is still 12 point not 18.