I don't know, but it's something I've been wondering for a few years now. I, too, have a Cannondale F5 (but from '07). I'm like a slug on that thing, but can ride my friends Salsa mtn bike (that is about the same weight) with SO much less effort. I'm perplexed. The shop I bought it from fit me to it so I'm certain it's the right size (they've fit my other 3 bikes just perfectly).
Makes me wish I'd never gotten rid of my old crappy rigid Trek mtn bike....