Este vem diretamente do livro:
And such bees! Bilbo had never seen anything like them.
"If one was to sting me," he thought, "I should swell up as big again as I am!"
They were bigger than hornets. The drones were bigger than your thumb, a good deal, and the bands of yellow on their deep black bodies shone like fiery gold.
"We are getting near," said Gandalf. "We are on the edge of his bee-pastures."
Quanto à razão, os animais de Beorn são anotados no livro para serem um tanto especiais:
At any rate he is under no enchantment but his own. He lives in an oak-wood and has a great wooden house; and as a man he keeps cattle and horses which are nearly as marvellous as himself. They work for him and talk to him. He does not eat them; neither does he hunt or eat wild animals. He keeps hives and hives of great fierce bees, and lives most on cream and honey.
Não há outro motivo dado, e devemos ficar satisfeitos com a explicação de que Beorn é uma pessoa mágica ("sem nenhum encantamento, exceto o seu") que mantém animais especiais.