ways to increase Animal Companion Size

[Deadkitten]

What are the ways that one could Increase an animal companion's size?

I am trying to get a colossal animal companion by using the Mammoth Rider prestige class from Pathfinder.
I honestly just like the idea of having a colossal wolf animal companion

[sleepyphoenixx]

A Wu Jen based Arcane Hierophant can cast Giant Size on his companion familiar.

[Khedrac]

Animal Growth (http://www.d20srd.org/srd/spells/animalGrowth.htm)

Remember the companion does not get the animal advancement size increases - that is a separate progression.

[Biffoniacus_Furiou]

Three levels of Psychic Warrior with Practiced Manifester can use Expansion to make himself and his animal companion grow two size categories, assuming magic/psionics transparency. Your companion should be huge size from mammoth rider, so this will increase it to colossal.