The headlines read: "Small Medium at Large." This type of joke is called a pun. Small, medium and large are all common terms of measurement. A fortune teller or psychic is sometimes called a medium, and the common phrase "at large" is often used to indicate an escape from prison.
The humor in puns relies on a word or words having more than one meaning. The surprise of seeing what was thought to be the meaning of the word inverted or changed to mean something else gives the frisson of enjoyment. Puns are one of the oldest forms of humor and have existed since at least the seventh century B.C.