The answer to that question is a different, simpler breed of mobile fortress -- one not designed with the elaborate complexities of a vehicle intended for long missions. Instead, I envision the A100 series as a line of short range mobile stations, utilized for transport, communications/sensor relay, and (in this case) battle, and designed to work with small crews for short term missions that don't require them to bunk on-board.
The interior from a distance. An immediate difference from the A200 series (beyond the size) is that the command level is on the second floor while engineering is on the first. Whereas the survival of the crew was the highest priority on the A200 series (thus the command center was placed safely behind the deflector array), on a battle vehicle, the first priority is to keep the guns firing, so engineering is placed behind the deflector array, where a lone surviving engineer can get in a few more shots after a critical blow to the main section.
Engineering level. Whereas the A200 series Engineering level is a lot more open and versatile, the A100's is streamlined for the sole purpose of war. As a result, there's a greater emphasis on refined user terminals than on access to raw hardware.
The Command Level. Note that the fortress is designed to work with only three officers: a captain/driver and two gunners/tacticians who can help inform the captain's decisions as he maneuvers the station against an enemy.
One of the tactician stations up close.
The captain's station up close. He's a little too close to the main viewscreen. Still trying to find a way to correct this without losing access to the lower level.