I was thinking the same thing. Has to be some movie magic going on there. In the stills from the movie that show them coming out of the airlock hatch on the planet, the mock-up they're climbing into and out of is not a full-size representation of the craft, and I believe this is why he used the term 'miniature.' Even so, the difference in scale is negligible in this context, and there would still not be the amount of interior volume depicted in the film.