The simple explanation is that a horn is most efficient to Fc if the system resonance is close to the Fc. When system resonance is below Fc you push it up higher by making the compression chamber smaller. This ties in with why you don't want to start with a driver Fs below Fc, because the acoustic impedance load of the horn and the front chamber push the effective Fs down, and can make the size chamber required to get system resonance back up near Fc impractical or even impossible.