- Yes, the ability score changes from the size increase is included in the changes mentioned in the animal companion description.
- Increasing size always grants a -1 to hit and AC, and a stealth/fly penalty. This you need to add yourself.
- Increasing size grants a +1 to CMB/CMD.
- In rare cases (such as the Ape, and other upright creatures) increasing size grants reach.
- Increasing size increases carrying capacity.
- Natural armor of companions is a mix between fitting their monster counterparts and balancing their statistics (not that it always works). The wolf companion as an example gets +4 NA at lvl 7, which is more than the normal wolf (having +2) at large size (+2 more).
- Reduce animal works fine. It effectively get -2 damage (on average), +2 AC, and the skill bonuses (single attacks, secondary attacks, and feats might affect these numbers).
Notice that animal growth is quite good, as it (unlike enlarge person) grant a significant str and con bonus.
- An amulet of reduce animal would cost 12k yes, for continuus use. You could choose to make it command word based and limit it to a number of charges per day.
For 6480 you could have a collar with reduce animal 3 times a day, 3 hours per time, requiring you to use a standard action to speak the command word.
The short answer is: because the devs said so.
Although the rulings on ACs is at times a bit clouded, they are still rules - nobody questions why a fighter has full BAB, right?
Concerning the individual questions:
Since it mentions modifications to str, dex, con and natural AC I assume you do not change these again for the new size.
Correct. The changes to stats mentioned are the only ones which have to be applied once the AC evolves. Here, the specific rule set for ACs trumps the generic rules for monster creation/advancement.
It does not mention a to hit penalty or stealth penalty for the new size, do these change as well correct?
Changes to to-hit, CMB/CMD, and armor class are not mentioned but have to be applied.
Same goes for changes to attack dice (if not explictly mentioned).BTW: there's a penalty to stealth for being large? Cool, never noticed - thanks!
Why do some animals only gain +2 natural AC? (wolf)
See first sentence.
On a side note: the AC stats and rules simulate the monsters you find in the bestiary, i. e. the large wolf tries to emulate the Dire Wolf (I believe). The starting statistics + the changes due to evolvement at druid lvl 7 should equivalent the Dire Wolf (roughly at least).
Does the reduce animal spell give you all of the benefits of a smaller size for the minor cost of 2 str and and smaller weapon die? These would apply to stealth as well correct?
Interesting thinking. But, yes. Once you got your large wolf, you could cast reduce animal on it and receive (most of) the former benefits and what's written in the spell (-2 to Str, +2 to Dex, etc.)
Dunno about the amu.
EDIT:Ninja'ed.
Ruyan.