I intended to play tag battle with my familiar, taking an arbiter which is the most resilient option available with regeneration and a buckload immunities thanks to constructed.
However, when I entered it into hero lab, and applied monstrous physique 3 on him, bumping him from tiny to huge, I saw his HP going from 33 to 73. That's when I read more carefully the constructed entry.
Although inevitables are living outsiders , their bodies are constructed of physical components, and in many ways they function as constructs. For the purposes of effects targeting creatures by type (such as a ranger’s favored enemy and bane weapons), inevitables count as both outsiders and constructs. They are immune to death effects, disease, mind-affecting effects, necromancy effects, paralysis, poison, sleep, stun, and any effect that requires a Fortitude save (unless the effect also works on objects, or is harmless). Inevitables are not subject to nonlethal damage, ability damage, ability drain, fatigue, exhaustion, or energy drain. They are not at risk of death from massive damage. They have bonus hit points as constructs of their size.
So constructed is an ability based on subtype (inevitable), polymorph spells don't say they change types or subtypes (please point me the relevent rule if I'm wrong). So changing the arbiter's size gives him a flat HP bonus.
An arbiter is an outsider, thus proficient with martial weapons, giving him a greatsword and bumping him to huge, with lead blades thanks to a wand, we have an almost unkillable (anarchic only) juggernaugth, with HP comparable to an average fighter, that deals 6d6+10 (~31) + weapon enchancement. Add hour/CL buff on him (false life, mage armor ...) if need be.
Am I getting something wrong or is it that good ? Especially considering that this guy can fight in a cloudkill, with mirror image, blink and everything his master cares to give him.
Seems to be hero lab error.
Arbiter has 15 hp. Making it huge would give it 40 more hp for a total of 55. Which is irrelevant, however, because familiars always have one-half of their masters' total hit points - increase in hit points of the base creature have has no effect on master's hp and thus does not influence the actual hp of the familiar.
I think hero lab takes it as an enhancement over the half master's base HP, much like false life temporary HP. But I think I will override his HP with master/2 at all times like you suggest, more fun avoiding him being knocked out, he is tough enough already.