In my case it was also on small bases (in terms of spawn points for aliens): one hangar, the lift, and then just a straight line away from the lift, to get them no means of outflanking you. I think, it comes with the lack of space to spawn. Just one hangar it is, for all enemy units, not counting the lift (which is always present).
Which brings me to a question, I've asked myself a while ago: What's happening, when there's no hangar at a base? Let's imagine a radar outpost: the lift, 1 x stores, 1 x quarters and 1 radar of some kind. Where will they spawn? Spawning only at the lift will make that thing quite cramped, especially if there's one of those big invasion platoons, consisting of 3-5 2x2 units and ~25 further 1x1 units.