But I did.
Resizing the fonts is possible and actually quite easy. I only resized custom fonts I added, no system fonts (which also could be loaded from /fonts , i just didn't try to modify them).
Use fontforge (the 7z pack is fine) and work alongside the following tutorial.
http://forum.xda-developers.com/showthread.php?t=990853
The pictures are missing, but you can actually leave all checkboxes as they are ticked by default, just deselect the validate one in the create font dialogue. The actual menu selection is called "Scale uniformly" and dont forget to save out as a ttf.
And back up your old ttfs before doing so.
edit: Oh, yes, and adjust the size of your Regular font first and compare on a page with regular text as well as some italic lines. After a few ;fc-caches you should have the percentage that fits your needs.
And in the case your Kindle refuses to boot (was a risk in the past, when modifying /font fonts), just connect it via usb and delete the ttf file you just modified, then hard reboot.
edit: one more thing, scaling steps on how big a font appears on the Kindle can vary quite a bit regarding the step up or down you tage percentage wise. So before you complain that it isn't working, try reducing the size radically (i.e from 100% to 20%) and load that font onto the KT (then ;fc-cache) just to see, that it is actually doing something. Then continue to find the right increment.