Little Big Brother

How can you tell which sibling is older on sight? Common sense and Captain Obvious say the taller one is older; after all, he (or she) has had more time to grow. But sometimes, due to circumstance or just Rule of Funny, the younger sibling will wind up the taller. Expect hilarity to ensue when people identify the wrong sibling as the elder...

Anime and Manga

 * Fullmetal Alchemist's Alphonse Elric had been taller than his elder brother Edward since childhood, but when he loses his body and is bound into a seven-foot-suit of armor, he towers over Ed, who is a hair under five feet (and very defensive when this is pointed out). Due to his new metal form, he often gets mistakenly attributed with his brother's title of "Fullmetal Alchemist". At the end of the series,.
 * Fairy Tail: Elfman is the younger brother to Mirajane, but is incredibly protective of her, to the point that it's almost ridiculous.
 * It's a case of Little Big Sister in the Hentai manga Take on Me, where Hikaru -- already a Huge Schoolgirl -- turns out to have a sister in her mid-20s who due to a medical condition looks like she's twelve.

Literature

 * Harry Dresden, at six and a half feet, towers over his older half-brother Thomas, who's a hair above six feet.
 * The Gruffs play with this one. The fairy tales about the Billy Goats Gruff are about these guys, including the "my big brother will get you" shtick, where each older brother is exponentially bigger.

Live Action Television

 * Supernatural: Sam Winchester is insanely tall, while his big brother Dean is extremely short. Sam even teases him about it once.
 * In Kaizoku Sentai Gokaiger, we have the Sneak Brothers, the elder of which is a little red spiked skull, small enough to be held in his "little" brother's hand.

Video Games

 * Even though he's shown as the cowardly one of the duo, Luigi is still the tallest of the Super Mario Bros.. Hell, in the animated adaptation of Super Mario World he's the Trope Namer of this.
 * Donkey Kong Country: Kiddy Kong
 * The Brothers Guardian Force from Final Fantasy VIII consists of two minotaur-like...um...brothers. The elder one (who doesn't even come up to his younger sibling's knee) even lampshades the party's, and more than likely the players', expectation when he first appears.

Western Animation

 * In the Regular Show episode "Don," the eponymous character is introduced as Rigby's hypercompetent brother and an endless source of frustration for him. He has a deeper voice, acts more mature, and looks like a grown man in a raccoon suit. It turns out that Rigby resents him because he's the younger brother, and people have always mistaken him for the older brother because he's taller than Rigby.