The biggest GV action is usually found in and around known spawn sights or spawns that are relatively close together, hundreds if not thousands of GV kills are registered at places like V85 every day yet bases where there is a large open space between spawns are mostly deserted and unless the town is within 4 K or less, it is likely not going to be attacked by ground forces!
Wirble's are also readily available at spawn camps when air craft come in to drop bombs due to the short distance to the fight
Breaking a spawn camp can be a lot of fun as well and unless your upping tigers or something perked a good bit, your not losing anything,,
GV's are also the easiest to remove from the game play by simply dropping one hanger at the corresponding air base or at the most, 4 hangers at a V base,
Making the spawns larger might help But I'd suggest something more like a secondary spawn site,2 or 3 k farther back and away from the primary to help expand the area to spawn in.