The New York Times has a graphic of the heights and ages of the presidential candidates for the last 29 elections. Apparently when there was a significant difference, height won out 17 out of 23 times, and girth won out 18 out of 24 times. It goes a long way towards explaining the Taft presidency, but it may have more to do with sartorial details than the Times realizes. We don’t remember the last president big enough to bust out the double-breasted suit, but we can’t think of an outfit more presidential than that.
Of course, Obama’s just a hair under 6’2”, more than half a foot taller than McCain, which might explain some of the recent poll numbers.