Orwell says in Chapter Two that the pigs "were generally recognised as being the cleverest of the animals." Thus it is natural that they would have been the ones to formulate the philosophy of Animalism and lead the Revolution. But pigs are also associated with greed, and Orwell often emphasizes their size, Squealer's in particular, to portray them as gluttons. Given Orwell's overall project in Animal Farm, i.e. to satirize the Revolution and rise of Stalin in the Soviet Union, it makes perfect sense to portray the leaders of the Revolution as pigs. Orwell also may have wanted to suggest something about human nature, or the nature of political power in general. Like pigs, those in power will always be greedy for more.