What's with the big hole in the middle? Could it not have been filled with something like cargo or living quarters or was it just a design choice?

It's a design choice and there are practical reasons for it as well. In our Shipyard post on Ship Mass, we indicate how we derive ship masses from their geometry. The Hammerhead is pretty fast for its size, since one of its duties is to help screen and protect larger ships from fighter attack. An increased internal volume - even from "filling in" the negative space, would add mass. In-lore, the UEE has ordered a lot of Hammerheads - thousands of them - for a combat-dedicated role. Adding mass for off-mission amenities wasn't deemed an effective design choice by the UEE Military.