Expansion groups and attack groups are totally different things. Settings under expansion only affect the expansion mechanic.
Attack groups are collecting, while the spawner consumes pollution. They have a gathering place near the nest, many spawners share a single gathering place.
If you hover over a spawner, you can see in tooltip, which types of biters can be produced and how much pollution is consumed by doing that (based on evolution)
The "attack cost modifier" directly controls, how much pollution is necessary to "produce" a biter.
Deathworld settings have them lowered to 50%, so you get more biters per pollution.
When the attack group is going to attack is a bit random, but the collected attack group gets dispatched after a certain time has passed. If you produce less biters, the attack group should be smaller, but it still might vary depending on the amount of nests and the random time, when they start the attack
So, I would also propose, that the best bet is to increase the attack cost modifier.
Evolution is another factor, where the biters themselves get stronger. It's a number between 0 and 1 and can also be read by hovering over the spawner. f.e. medium biters are showing up at 0.25, but they also consume more pollution to get produced. You could say, that biters get more efficient with higher evolution by getting stronger for less pollution.
Evolution settings affect, how evolution gets increased. (It can't be decreased). Pollution is double-edged as it increases attacks but also increase pollution. For deathworld the time factor is a lot harsher, so you can decrease or zero that, to don't have time pressure. And of course destroying spawners increases that. The evolution factor is growing a lot faster in the beginning, but it stagnates, the higher it gets. So you are quite fast at 0.5, but it takes quite long to get to 0.9.
Expansion is, when the biters spread and build more spawners, so you basically have to wall areas off, if you don't want new nest to spawn. But expansion can be turned off. F.e. Railworld preset has it disabled. You can clear areas of biters and they won't come back ever.
If you clear the same area multiple times, you always increase evolution, so it might be wise to clear them, when you are able to fence the area off.
everything is also explained in wiki: