I'm not familiar with this program, but it probably has to do with remoteness too, not just size. For instance, as svpii recently posted, HKY is losing their only airline, but they are only an hour away from CLT anyway, so I would bet that the DOT would not cover them for this since they will really still have access. How far is AHN from the next nearest airport out of curiosity?

I would bet the criteria is more focused on remoteness actually, and assuring that if folks are about to lose the only air service within, say 2 hours drive, the DOT will fund it ... and of course, airports with only one carrier to begin with are most likely to fall into this category.