Giant animals, insects, and humanoids are physically impossible.  However, such critters are a staple of science fiction, fantasy and horror genres.  A referee may just handwave away the impossibility of such beings, but for those of a more "rational" bent, we offer the following rationale for the existence giant critters.

Empathic abilities are not reliant upon intelligence, and may manifest themselves naturally and subconsciously.  This is the case with giant critters.  Giant critters have an innate Change Environment (Extended) ability (identical to the Sorcerer's skill) that extends only to themselves.  Typically, this simply affects gravity, allowing the critter's legs to support its own weight.  In the case of some critters (especially giant insects), it also creates a dense envelope of air around the animal, allowing it to breathe where it otherwise would not be able to.  This also gives the animal an additional AV of 1.

The "Giant Critter Rule" can also be used to rationalize the flying abilities of some creatures.  Such critters have an innate Sorcerous Flight ability.