I'm a mean beast and cut it very fine

I'm reading The Wind in the Willow and has some question.

'What's inside it?' asked the Mole, wriggling with curiosity.

'There's cold chicken inside it,' replied the Rat briefly;
'coldtonguecoldhamcoldbeefpickledgherkinssaladfrenchrolls-
cresssandwichespottedmeatgingerbeerlemonadesodawater—'

'O stop, stop,' cried the Mole in ecstacies: 'This is too much!'

'Do you really think so?' enquired the Rat seriously. 'It's only what I always take on these little excursions; and the other animals are always telling me that I'm a mean beast and cut it very fine!'

What does this sentence means "I'm a mean beast and cut it very fine!"

Thank you very much