> 1. Why did it work "out of the box" with readable font sizes for the
iRiver?

AFAIK, the same default font is used on all targets, but according to
http://www.rockbox.org/twiki/bin/view/Main/DeviceChart, the H10 20GB has a
screen resolution of 160x128 (~4 pixels per mm vertically), where the
Gigabeat F40 has a resolution of 240x320 (~7 pixels per mm).

So, the same font will appear about twice as tall on the H10 as on the F40.
Apologies if my early-morning thinking is askew!

-- 
pondlife