[3.5] increasing druid's companion's HD

[Raezeman]

when a druid reaches higher levels, his animal companion receives bonus HD. I take it that for this can bring the animal into a new size category, as stated in the advancement line of it's description in the MM. So now my question is:

The animal get's a bonus in natural armor, both due to druid level as due to new size category. Do these bonuses stack?

[ArcaneGlyph]

"Natural armor bonuses stack with all other bonuses to Armor Class (even with armor bonuses) except other natural armor bonuses." So you would pick the higher bonus and apply that to the AC.

[hymer]

I wouldn't go with the initial assumption, actually. A lot of things are spelled out under HD advancement for druid companions, which is also found in the 'advancing monsters by HD' bit in MM. But not the size increase. Increasing monsters by increasing their HD also seems capped at x3 HD. A wolf could go way beyond that as druid companion.
I think those two advancements of creatures are separate, if similar.

[Dread_Head]

I'm pretty sure increasing an animal companions HD using a druids advancement doesn't cause it to increase a size category.

However if it did then the NA increases would stack as they come from different sources and improve an existing NA bonus.