I thought the "the biggest thing to happen to iPhone" tagline is a pretty cute pun referring to the increased physical height until I read this thread. Seems like I'm the only one thinking that. Oh well, tagline done terribly, I guess.
I thought so too, then I saw that the marketing department decided to then prattle on about reductions in height and weight. They also show the phone side-on and there's no physical comparison to the earlier model showing the larger screen. The message wasn't very clear, and seemed to contradict itself.
In fact, it seems to be a play on words that the 'biggest change is the fact that it's so thin' and not the 'biggest change is the biggest screen'.