I'm not kidding!!
Try it.
I went from 42s to 44s (center), and my bike feels a little faster when I ride off the saddle.
And it's not just when I'm really ripping on the bars, but even when I'm just 'leaning' on them when I rock from side to side up a hill.
I figure it's due to the perception that there's a little extra leverage, even though there's more than likely a little more flex