Sass. I always used to wonder, "So what? What if he is just a shorter-than-average Stormtrooper?" I wonder what would've happened to Leia then, lying there seductively, hand on hip, confronting two armoured troops.
But then I realised that the Empire probably has height restrictions when hiring new staff. I mean, they only hire poncy sounding white dudes for a start, so what's to stop them from only hiring six foot plus Stormtroopers?
Of course, the line makes a lot more sense now that we've seen the prequels. All Stormtroopers are clones, thus, the same height. When Leia sees Luke in Stormy armour, she knows something's up. That still doesn't excuse her for lying all seductively like that, hand on hip, lips pursed, alone in a prison cell...
Incidentally, there's an Aussie Star Wars fanzine that uses this quote for its title, although they temporarily changed the word 'Stormtrooper' to 'Battle Droid' (groan) when Episode I was released, and to 'Clonetrooper' around the time of Episode II. I'm not sure if they came up with anything witty for Episode III's release, as I stopped buying it once I realised all the news they printed (once a month) was avaliable on the web weeks ago.