hm
Bzzz... Wrong.
You can have two different spells give a similar effect without being identical.
And what does this have to do with this discussion? I'm not arguing that wildshape and animal growth are identical. They're clearly not.
That said, animal growth has a "size increases through magic do not stack" clause that in my opinion prohibits increasing druid's size further.
Wildshape is a shape altering magic, not size altering magic. The change in size is a side effect, not the purpose of the spell.
How does this matter?
It's a very simple binary question - is it magical - yes. Does it increase size - yes. Then the two won't stack.
The "purpose" of the spell is whatever the user wants it to do. One wizard could use fireball to blow up creatures, while another one could use it to burn dry leaves cluttering his yard.
The "effect" of animal growth is to increase size. The "effect" of wildshape is to change into an animal. When the animal is size large, it conflicts with animal growth "no stacking" clause and the animal stays large.
Nothing prevents a medium-sized druid from wildshaping into a medium-sized dog and then animal growing himself to a large-sized dog.
Wildshape and Animal Growth can achieve the same general effect, but they do it through different ways. Thus, they would stack.
If a spell says that the target is imbued with strength of a bull while another spell says target is consumed with strength-boosting rage, the two spells won't stack if they provide the same bonuses, even though they achieve said bonuses in completely different ways (one is transmutation while the other is mind-affecting for example).
Wildshape does not provide a named bonus to size. Animal growth does not provide a bonus to size. In fact, they don't mention any bonuses to size at all. Does sprited charge stack with a critical hit? There is nothing to stack - the two multiply things, they don't provide bonuses.
Just like the strength bonus from Bull Strength stacks with the strength bonus from Enlarge Person, and the strength bonus from Divine Power stacks with the strength bonus from Righteous Might.
Again - what does this have to do with anything? Neither wildshape, nor animal growth provide bonuses to size of any kind.
Wildshape is not a "Size Altering Effect".
Has your sized chaged? If yes, then it's is a size altering effect. It doesn't get any more basic than this.
You argue that this is a "side effect" of becoming a different-sized creature. So what? It could be a side, top, or down effect, but your size has *increased magically* and that's what matters. In my opinion size change is the primary effect because it helps grappling which is what I intend to use it for.
It changes you into a creature that happens to be a certain size. If you kept the same shape but increased in size, then it would be size altering magic.
Why does shape matter?
I'm not talking about size altering magic in terms of flavor. I'm talking about size altering magic in terms of rules. If a creature is one size and then through the use of a continuous magic effect becomes another (whether it grows while retaining the same shape, changing into another creature, or simply inhaling a whole lot of air and bouncing around like a beach ball), it chages size.
That said, I hope you're right. Hey, if designers think that huge druids with improved grab and +35 grappling modifiers at 12th level are balanced, I won't complain. I intend to play one.