Hi everyone,
I've got a druid in my group who wants his Roc animal companion to be gargantuan, and so I've decided to make a feat chain for it. What do you think? Are they too weak, too strong?

Improved Animal Companion
Prerequisite: Animal Companion (or similar) feature with an effective druid level of 10, large animal companion.
Benefit: Animal companion becomes Huge by application of the following statistical changes:
Str +2, Dex -2, Con +2, increase size 1 category, increase damage dice for natural attacks 1 category.

Greater Animal Companion
Prerequisite: Improved Animal Companion, effective druid level of 15
Benefit: Animal companion becomes Gargantuan by application of the following statistical changes: Str +2, Dex -2, Con +2, increase size 1 category, increase damage dice for natural attacks 1 category.

I'm worried about the fact that the size increases will confer natural reach on the animal companion (10 ft and 15 ft), and also the +3 increase to CMB that comes with the Greater feat. I'm also not sure whether I should build in a natural armor bonus to counteract the AC decrease due to size. These feats will be available to every animal companion that meets the pre-reqs, not just Rocs.

Thanks in advance.

If Pathfinder is anything like 3.x still, then there's far more powerful stuff available far earlier. If anything, I'd wonder if they're too weak. By level 10 and 15, you're going to come up against stuff all the time that a little more reach or CMB isn't going to help with much, if at all.