If you bring up the map, each glyph is a single overmap square, so a house or a section of street, or a shop. Also roughly 70 player-sized squares.
The alternative is to have deer and squirrels and crap crammed in with the zombies, and also it would make even more animals, fungals, triffids, etc spawn, it’s part of the self-limiting of dynamic spawn.
Finally we really don’t want to mess with it that much because what I really want to do is just replace the whole thing.