From what I understand, the Swarm mechanic, while it looks like the other units are attacking, they're not. Instead, they're lowering the target's defense by distracting it. I'm assuming that it will drop it into the negatives causing the damage to increase, because sometimes I get some pretty high damage numbers with it. I would like to see the math involved, or at least a more detailed explanation, because sometimes I see larger damage bonuses than other times.
I've noticed some pretty high damage numbers as well. The more of the same unit I move in, the higher the damage was.
I do like that there is a bonus for adjacent units, but I wonder if it's a bit overpowered at the moment? Not that I mind when it benefits me. 