The pigs do begin to appear to be more human, demonstrating the type of leaders they have become. The significance behind this, however, is less literal than that. It is important to note that Orwell chooses to portray the pigs in this way at the very end of the book to achieve one of his themes: that power may shift after a revolution, but oftentimes things will stay just about the same for the proletariat, or working, class. Power corrupts; as it did Mr. Jones in the beginning of the book, so it has the pigs. Life for the rest of the animals remains the same, or worse.
The major change in the pigs' appearance comes in the very last pages of the book. By acting more and more like humans, and through taking on human characteristics (which range from running things to living in the house), the pigs eventually become indistinguishable from humans. Squealer and the other pigs even start walking on their hind legs, until, as the final line of the book says, "The creatures outside looked from pig to man, and from man to pig, and from pig to man again; but already it was impossible to say which was which."
Near the end as the animals look through the window they realize they couldn't tell the pigs apart from the humans this represents how the pigs have changed and became more humans. They had started wearing clothes and walking on two legs.
the pigs start to act more like humans
they begin melting into human form and they become the exact same as humans. i'd give you a longer answer but i don't have much time.
Animal farm is all about power and wealth. In the book, think of the animals as 'poor working people'. And think of the humans in the book as 'rich powerful people' (the owners of property, money, power etc).
After the worker's revolution on Manor Farm, the pigs begin as normal animals, (ie poor workers) but they become the leaders on the new Animal Farm and the advantages of power changes them. Little by little they begin to change from animal to human (ie from poor to wealthy elite)
They separate themselves from the other animals and move into the farmhouse. Gradually they betray their animal comrades and adopt a human lifestyle. By the end of the book, while the normal animals are cold and hungry, the pigs are wearing clothes, sleeping in beds, fat and learning to walk on two legs. At the very end of the book they have dinner with humans at the farm's dinnertable and the pigs and humans look the same. In other words the pigs have completely changed from animals (poor workers) to humans (rich owners).