In RM2, it was Tiny Bite and Tiny Grapple. You are correct there, but you state that it doesn't make sense. What is the creature producing those crits?
For swarms of small insects like bees or spiders, I let the players roll the attack, subtract the DB, then look at the resulting damage. If the result is something like 14 points damage and each creature has 2-3 HP, then they killed 5-7 creatures. I just spread the damage out over the bodies, killing off as many as needed to equal the points of damage delivered. Other times, if the attack results in a hit with damage, I roll D-10 so see how many creatures have died.
There is usually more info in the creature description describing how to resolve the creature/swarm attack. Sometimes there is a different bonus if say 10 or more rats attack as a pack, rather than resolving 10 individual attacks which can take a long long time.