Animal Growth Spell
Animal Growth Description: A number of animals grow to twice their normal size and eight times their normal weight. This alteration changes each animal's size category to the next largest, grants it a +8 size bonus to Strength and a +4 size bonus to Constitution (and thus an extra 2 hit points per HD), and imposes a -2 size penalty to Dexterity. The creature's existing natural armor bonus increases by 2. The size change also affects the animal's modifier to AC and attack rolls and its base damage. The animal's space and reach change as appropriate to the new size, but its speed does not change.
More: D&D Spells for Animal Growth