Georges Méliès had been using the trick of multiple-exposure photography (or superimpositions) since at least "The Cabinet of Mephistopheles" (Le cabinet de Méphistophélès) (1897) (a lost film). One of his earliest and best surviving examples of this trick remains "The Four Troublesome Heads" (Un homme de têtes) (1898). By 1901, he and nearly every other filmmaker had employed the trick in dozens to hundreds of films. He took the gimmick about as far as he could in this form with the sevenfold exposure of "The One-Man Band" (L'homme orchestre) (1900). In other words, the effect could use some new life, even by then. Méliès gave it just that with "The Man with the Rubber Head".
It appears to be the first film, or at least the earliest surviving or available one, in which the director discovered the fantastical and amusing possibilities of enlarging or shrinking one exposure while not the other. He loved it so much that his next two films listed in the Star catalogue (and also available on the Flicker Alley set) employed the same technique. As usual, other filmmakers, too, were quick to imitate his work; one example might be the giant ogre in Walter Booth and Robert W. Paul's "The Magic Sword" (1901) (although it's contested who did it first - see my review of "The Magic Sword" for more details). For the growing and shrinking head in "The Man with the Rubber Head", Méliès moved towards and away from the camera via a chair and pulley for one exposure, which was to be joined with the other of the chemist (also Méliès) and his assistant. All of this was done in-camera and thus required precise execution. Méliès's body excepting his head was covered by black as to prevent exposure (i.e. masking). A stop-substitution (or substitution-splicing) was used for the explosion of the head-this trick was even more tired than superimpositions.
It's also worth noting that this is, in a way, a rare and early use of the close-up by Méliès. It's actually one of the earliest close-ups of the face that I've seen or know of (not counting the medium close-up popularized by "The Kiss" (1896) - that not being as close of framing). Other early close-ups tended not to be of the face, such as the magnified objects in George Albert Smith's "As Seen Through a Telescope" and "Grandma's Reading Glass" (both 1900). Méliès's close-up is also interesting because it's the reverse or ersatz of a dolly or trucking inward shot-moving the actor rather than the camera. The dolly shot becoming a close-up was later used as the attraction in single-shot Biograph films "Hooligan in Jail" (1903) and "Photographing a Female Crook" (1904). Méliès's close-up also remains within the long-shot framing of the outer exposure.
