I don't have first hand experience with Afghanistan, but I'd assert that just like there are calculations involved in employing combat power in a "conventional" sense, the same holds true in an irregular environment. The basic answer to your question are the fundamental planning factors on what is required to secure the force, whether it be in a small COP or a large FOB, or "going mobile" ouside of the wire.
There is an advantage to the large FOB in that for a set investment in FOB security and logistics, you may be able to generate much greater aggregate combat power outside of the wire at any one time. The obvious disadvantage, as you point out, is that fewer large FOBs means less density of presence and greater distance any force must travel outside of the FOB to cover an entire area. The lower density drives the greater travel distance and results in the "commuting to war" charge as forces spend as much time getting to/from all points of their operating area as they do actually operating in it.
I don't think there's a single answer to your question, but its METT-T dependent. However you highlight some real considerations when choosing a laydown. Yes, big FOBs can be bad, but there are advantages. You should probably consider a mix of solutions. Even though COIN may be all about being out with the population, there are hard calculations in how you can generate the necessary persistent (or episodic) presence.
Happy Birthday Marine!
Phil Ridderhof
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