The reasons the pro cyclists tend to ride smaller frames than us mere mortals are as follows, pick any or all of them:

1. Less carbon lighter frame
2. shorter tubes means stiffer frame
3. All pro tour bikes need to be available for sale, as such many of the frames made for mortals have higher stack heights, so if the pro chose the correct size frame he may not be able to get the bike to his correct fit specifications. Many of these guys ride pretty severe saddle to handlebar drop to get as low and aerodynamic as possible.