The SNV Horizon is the first of the Horizon-class carriers. Unlike other carriers in use by the SDF Navy the Horizon-class focus more on carrying large groups of cargo shuttles and troop transports.
This is the primary reason why the hangar area is much smaller and also why the cargo compartment is much larger. In the Horizon's case it has been heavily modified so that it may carry the bulk of the Phantom Warrior program soldiers, turning the cargo compartment into a barracks, shooting range, training course and briefing room in one.