Animal Growth

When this spell is cast, the caster causes up to eight animals within a 20-foot-cube to grow to twice their normal size. The effects of this growth are doubled HD (with resultant improvement in attack potential), doubled hit points (except hit points added to Hit Dice), and doubled damage in combat. Movement and Armor Class are unaffected. 

In the reverse, shrink animal, reduces animal size, Hit Dice, hit points, attacks, damage, and so on.