I am wondering, why this particular rule is there. To prevent from changing a halfling into a T-Rex? Can it be ignored without negative effects?
SRD said:This animal can be any predator, scavenger, or omnivore whose size is within one size category of the base creature’s size (Small, Medium, or Large for a Medium base creature). Lycanthropes can also adopt a hybrid shape that combines features of the base creature and the base animal. A lycanthrope’s hybrid form is the same size as the base animal or the base creature, whichever is larger.