Need help balancing Druid Archetype Idea

[Jaycaldor]

This time around I wanted to kind of empower my Animal companion a bit and shift a bit of the focus towards it.

All the changes I wanted were already approved by my DM, but I myself think I should sacrifice class features for the benefits I made.

The main features of this Archetype of mine are:

• Animal companion starts one Size cathegory larger, If this makes it Large, further Size advancements are omitted.
• Animal companion gains Grab (Ex) and Swallow whole (Ex)(Creatures up to 1 Size cathegory smaller)
• At level 4, the companion gains Grow (Su/Ex) --> See below

Grow (Su/Ex): The Creature can double its Size for a number of rounds per day equal to 4 + its Constitution modifier (borrowed from Barbarian Rage, advancement is likewise the same). Acts as Enlarge Person with Animal companion as target.
I'd say the Druid uses a Standard action to activate this ability.

Still debating if it should be Su or Ex...

By the way, the Animal companion I have chosen is a Big cat

Now I ask of you guys, in what manner would you balance these changes?
Obviously Spells and Wildshape are the last thing I would want adjusted.
How would you calculate Internal damage/hp?
I got a few alright sets of numbers from scaling Swallowing creatures like the behir to my companion's HD.


Also, if you see where i'm going with this I would appreciate Ideas on how to make the focus even deeper while staying balanced. (Swallow whole normally is hardly the most useful thing in most situations).

[Jaycaldor]

Even though it seems useful, I wouldn't mind for example replacing the rake ability of my Big cat, but I'd like some general Solution as well...

Any ideas?

I also think the kind of action required to activate 'grow(Ex/Su)' should lessen with Level advancement.