The hobbit holes were made different sizes to allow the visual illusion of small people and big humans. This meant that everything to do with the holes was made at that scale. They had children actors wearing adult clothing, working around the holes to give the right perspective.

Some of the holes were full size for parts of the movie.

This gave us an odd illusion at one point – we had got used to the tiny size of the holes, then a full sized gardener came out of a full sized hole – and immediately looked like a tiny person.