Hi!
I think, nothing special here. But have a look at the screenshot. Nearly half of the AREA ist white(margins) if you sum up all 4 sides and compare it with the area filled with text.
In my opinion a maximum of say 5mm - 7mm is ok. That's the case for example in Koreader.
And to demonstrate it (yes, I've too much time today...):
This should demonstrate how rediculous this options are, esp. on a 6-inch-reader.