If you look at it on Google Earth, you'll see that it's fairly spread out. There are a large number of separate warehouses (around 100?), and each warehouse has large earthen banks around it, then a large amount of empty space. That's intended to contain fire & explosions, so if something goes wrong you might lose a single warehouse but they don't spread to other parts of the complex. That's clearly not working very well. But those earthen banks and empty spaces take up a large amount of space, probably over 90% of the site.
So it's fairly spread out, even though it was at a single site.
People tend to object if you build an ammo bunker next to their house, so it's always going to be awkward to get more sites. You also need to secure ammo bunkers very well, so having less sites lets you have better security for the same money, or spend less to get the same level of security. Making a base 4 times bigger only doubles the perimeter fencing needed, probably doesn't change the number of guarded gates you need, and the number of guards needed only goes up a little bit since the guards are mostly there to man the gate and to react to an intruder. So having a small number of large sites does make sense.