They are roughly arranged so as to keep the chain line straight-ish when you use the gear combinations that develop the most power via a human's legs.
Having said that, they could both be flipped and that would still be true - smallest on the outside at the front -but then two things would happen
*The small cog at the front would really eat your socks/jeans etc
*The large cog, now on the outside at the back, would have a lot of leverage perpendicular to the bearings of the free hub, and be mounted furthest from them [seeing as small driving large will develop the most mechanical advantage]
This brings me back to the question from yesterday's sprocket thread - does the force on the teeth of a chainwheel or cassette sprocket have to do with the gear combination driven, or only the force input from the pedals/crank arm length.
Maybe I'm thinking too much too.
EDIT: awww heck