One lore-rooted explanation is that Installation 07 was formerly a "Greater Halo," which were 30k km in diameter. All of the other Greater Halos were lost aside from Zeta Halo and Zeta was, at one point, forced to jettison a bunch of its material (establishing the modular nature we're seeing here), reducing its size to the 10k km diameter of the Lesser Halos. It joined the six Lesser Halos as the seventh of the set.

There's a handful of info on what differentiated Greater Halos from Lesser Halos but 343i has left plenty of room for the mystery of what makes it so different.