Hi Kenneth,
I believe the reason for the large margins are based on readability (long lines are harder). Having said that, I've also found them a bit 'odd' and maybe that's because I'm so used to Microsoft Word's default margins of 1in all around.
Anyway, if you prefer to specify margins directly, rather than calculating them via \textwidth, you can also use the geometry package, e.g.
#+LATEX_HEADER: \usepackage[various options]{geometry}
Cheers,
AK