Suggest ability to adjust settings ingame. when i installed the mod i immediately had giant biters attacking my outer walls.
Could you provide a little more information about the game state(game time or evolution factor)?
What settings are set before starting?
If the size of biter bases is set greater than 100%, it increases the chances for giant units to appear.
Alternatively you can try a mod that adds air purifiers. This will reduce pollution. The chances of giant units appearing in groups will also be lower.
https://mods.factorio.com/mod/electric-air-purifier