Simple Q&A D&D 3.5 (by RAW) XXII

[Urpriest]

Q 1176

If a Druid's animal companion reaches sufficient hit dice that it advances in size, can it grow to the next size category? All logic seems to say yes, but I think I read somewhere that animal companions can't grow in size due to bonus hit dice (though, in turn, I can't seem to find where I might have read that).

A 1176

No. A Druid's animal companion is typical of its kind. It cannot be a creature that has been subject to monster advancement. While animal companions gain bonus hit dice, this is an entirely different mechanic and there is no reason to conflate the two, since the bonus hit dice are not limited by the animal's advancement entry.