Patryn is correct. The clause on Animal Growth is meant to prevent, for instance, a Polymorphed cleric from having Righteous Might and Animal Growth on her at the same time. Basically, consider any spells that give a "Size bonus" to not stack, just like any other sort of bonus wouldn't stack with another of the same sort. Polymorph/Wildshape does not give size bonuses. It just sets your stats to the average for a rather wimpy (rolls of all 10s or 11s) member of the species in question. Then Animal Growth gives a size bonus, which stacks.Patryn of Elvenshae said:It is allowed.
Some people don't like it, and argue that it's overpowered. Their ruling against it centers on the fact that multiple size-increasing magics don't generally stack with each other.
Those who believe it does work point out that polymorph and similar effects don't really change your own size so much as cause you take on another form - which may or may not entail a size change. In plain English, you didn't change your size to Large, you turned into a normal-sized bear.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.