Since I've never owned DX or DXG or even Kindle 2, I must ask you to refresh my memory: did those devices had user accessible "reader.pref" file at
/mnt/us/system/com.amazon.ebook.booklet.reader folder (where /mnt/us = "Kindle drive" = what you see when you connect Kindle to your PC; system folder is hidden by default) like Kindle 3 has had it?
It your device has that reader.pref file inside you will see "FONT_SIZE=25" (or some other value) you should try this:
1) Exit to "home" ie exit any book you where reading
2) Connect you Kindle to your PC, go to that location
3) Open reader.pref in some text editor (like Notepad++ if you're on Windows)
4) input value bigger than 72 so you have something like "FONT_SIZE=96". Save the file
5) disconnect you Kindle from you PC.
6) Don't open any book on your Kindle (if you do it will overwrite the change you made)
7) Restart you Kindle from Setting page
8) When Kindle finishes restarting open a book (
don't touch anything in Aa menu) and see if the text is now larger than that largest font size.
Check
this thread for some more info and screenshots of reader.pref contents>
If you've managed to get bigger text with that "trick" than you can consider making a permanent change to those 8 predefined font sizes in Aa menu by using
this hack. Check included readme file (and/or linked forum thread) if it support your Kindle's firmware version!