I'll cast the dissenting opinion on the manner.
If changing size wasn't an additional power above and beyond that of the basic wildshape, wouldnt it seem reasonable that there wouldnt be any need for additional powers recieved later on that allowed you to shift shape AND size?
Consider also that Rystil says "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." This is a pretty common explanation but one Ive always thought to be flawed.
Clearly though if you look at the animals you'll see evidence that size IS giving you a bonus. Consider these examples, taken from the SRD
ape (large) Str 21, Dex 15, Con 14, Int 2, Wis 12, Cha 7 vs.
baboon (medium) Str 15, Dex 14, Con 12, Int 2, Wis 12, Cha 4.
Ok, these are primates, but not identical so maybe thats why one is stronger, more nimble, and heartier than the other.
Black Bear (medium) Str 19, Dex 13, Con 15, Int 2, Wis 12, Cha 6 vs.
Brown Bear (large) Str 27, Dex 13, Con 19, Int 2, Wis 12, Cha 6
Closer species perhaps, but still not identical. Still, clearly being large in wildshape has benefits over being medium size.
Crocodile (medium) Str 19, Dex 12, Con 17, Int 1, Wis 12, Cha 2 vs.
Crocodile, Giant (huge) Str 27, Dex 12, Con 19, Int 1, Wis 12, Cha 2
Identical creatures. I dont think you'll get much clearer evidence.
FWIW, We ruled you COULD use animal growth to augment your wildshaped form IF you had shifted into a form that was different from your size. I guess in the end, this may just be one of those things that falls under Rule 0.
Good luck with whatever you decide.
The larger animal has higher str, dex, and con