I noticed that the game has difficulty distinguishing actual bases as often times purges would target a staircase, bridges and single foundations I have scattered around the map.
With this in mind, it seems like large bases are susceptible to having mobs spawning inside because the spawn distance to a particular foundation is still within the boundaries of these bases. This issue extends vertically as well. In my case, I have had forgotten tribe and silent legion mobs spawn inside foundations as well as on my 2nd floor in what would be geographically accessible bases.
In addition, I think it would be a good idea to implement additional settings such as a multiplier for purge meter gains as the numbers seem to be adjusted for larger servers in mind. This would help those playing on smaller private servers and single-players trigger purges more frequently.