"What has size got to do with hit dice except for realism issues?"

It's not a matter of realism, it is a matter of balance. Tiny constructs lack the bonus hit points because of size. Without all the spells and special qualities, this would be a significant disadvantage. The CR boost isn't from HD, which is where it should be. Generally this is bad design.

If you want realism, the party could try resonance frequency which would shatter the monster without the use of magic and would probably do enough damage to overcome DR 30/- and still kill it. No Save. Just massive damage. The blast radius when that happens would be interesting.

Wait, does bringing science in mean I've now killed a catgirl? Cool!

Debby