Why copes ideas about the evolution of tetrapods were considered a hypothesis and not a theory

Cope's Rule postulates that animal groups tend to evolve through time towards larger body size. Even though larger body size is often advantageous trait it isn’t always that way (larger organisms require greater amount of food, and can have problems when food is sparse).  

But this postulate isn’t theory, because body size increase through time, but the mean body size often doesn’t.

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