Mixed size wheelsets are popular with crit speed demons and time trialers who want more aero while keeping more control. These meet that philosophy with disc braking. That means getting a wheel up front that's deep enough to slice wind, but not so deep as to catch a lot of it and make steering a mess. Side wind affects the rear of the bike less, so people will go bigger there. Unofficially, we call these mullets - business up front, party in the back...