In How the Grinch Stole Christmas!, the Grinch’s heart is 2 sizes too small.
The phrase appears in Dr. Seuss’s 1957 children’s book, where the Grinch is introduced as a bitter, isolated creature living above Whoville. His undersized heart is a comic physical image for his hatred of Christmas and his lack of sympathy for the cheerful Whos below.
The story turns that measurement into its central transformation. After stealing the presents, food, and decorations, the Grinch expects Christmas to collapse. Instead, the Whos sing together even without their possessions. He realizes that Christmas means “a little bit more” than material gifts, and his heart grows 3 sizes.
A common mix-up is to remember only that the Grinch’s heart grows, or to confuse the starting number with the ending number. The original description is 2 sizes too small; the later growth is 3 sizes. The story was published in 1957, and later animated and live-action adaptations helped make the wording one of the best-known lines in holiday pop culture.