An interesting question arose this weekend and after searching the forums, I was unable to find an official answer. What happens to the equipment for an animal companion when it grows? For example, at level seven, a large cat (medium) grows to large size. If it was wearing +1 mithril chainmail barding, does the armor "magically" grow with the animal? Or does it have to be immediately sold for half-price and repurchased at full price including the adjustment/increase to account for large size? Or maybe you only have to play the difference?
Sure, the easy and harshest answer is the former one. It makes the most sense. However, this is a fantasy world where not everything makes sense. Maybe for PFS, you only have to pay the difference. The armor magically changing size seems the least likely action.