I agree that this was an oversight, with infinite hp bosses. When it had a finite amount, the damage reduction was necessary to slow the big guys inevitable demise.
There are ramifications for changing it now though. As in, more cores going out than expected. And we can't push the damage needed to get a core. So this will be discussed tonight.
After a quick discussion, we agreed that smaller pets should be given the chance to deal more damage. So the defense has been significantly nerfed, and given a moderate attack to compensate.