In 3.02, at 70, the pet tab (made visible with dedmon's macro) was showing 307 base stamina + 30% of my stamina.

I'm not sure how much health pets are supposed to get from stamina. I'm pretty sure I've seen them die from 2-3k damage when their stamina (by this calculation) should have been over 500.

Their armor also scales (helpful for tanking trash). Armor (at 70) is approximately 6700 + 36% of mine. Moonkin form makes a difference, but I don't know if an existing treant's armor will change if you shift. I tested at three armor levels, and did not get a straight-line on this, so it may be that some armor (ie MotW or Agility) is not included in the scaling.