Changing focus on a camera doesn't make objects larger or smaller, it just well.. brings the object into focus.
Actually, changing focus does make the scene appear larger or smaller, but only slightly. That's because it changes the distance between the lens and the imager/film, which changes the size of the image formed on the latter.
This is especially noticeable for macro lenses, which move out quite far from their "infinity" focus points, but still it's important in photometry. For example, it has to be taken into account in measuring the size of the earth that appears in the background of the photos the Apollo 17 astronauts took of each other next to the flag. The camera is quite close to the flag and astronaut so its lens was moved relatively far from the film to bring them into focus. This made the (out of focus) earth appear slightly larger than it would if the astronauts were taking a picture of the earth with the lens focused at infinity.
This isn't directly relevant to the present discussion of pictures of the earth and moon together from space well away from both, as the lens would simply be focused at infinity and left there. But it should be pointed out for completeness.