Improved Natural Attack + Swarm Damage

[Yogibear41]

Would taking Improved natural for a swam's damage boost it to the next HD value listed in the monster manual or would it follow a different increase?

For example if a 18 hd swarm (normal swarm damage 4d6) took improved natural attack, would the attack do 5d6 damage (for swarms of 21 or more HD) or would it do
6d6 damage following the progression in the Improved Natural Attack feat?

What would happen if the swarm's HD increased to 21?

[MisterKaws]

RAW-wise, it would follow the size progression chart.

RAI-wise, who knows? Ask your DM.