Hmm, I'm trying to reproduce using RText's built-in font resizing (Ctrl+F6 and Ctrl+F7 map to RSyntaxTextAreaEditorKit.DecreaseFontSizeAction and IncreaseFontSizeAction, respectively), and it seems to resize all tokens, even keywords (although doing this did help me to spot a real bug - those actions only affect the currently focused text area, not any others. The actions in RText's menu bar should affect all text areas. I'll need to fix this!).
Are you modifying RText's source and doing something with IncreaseFontSizeAction yourself? Are the current actions (found at View -> Font Sizes -> ...) not sufficient? Just curious if what you're working is a useful feature and could be added into the next release.
Check out where these actions are added to the menu bar in src/org/fife/rtext/RTextMenuBar.java
. Off hand, I don't see why how you're using the action wouldn't work, but then again I don't know exactly where you're adding this code into RText. Again, a sample program would be best to help find this issue. Perhaps you've found another bug, that only shows itself under certain conditions!