Sirius had always been bigger than him.
He was his big brother, obviously, and insisted on introducing him as my little baby brother Reg even to people they already knew. He would rest his elbow on Regulus's shoulder and muss up his hair and get things off shelves for him even though he could definitely reach and even if he couldn't, he could use magic couldn't he?
But he was bigger in other ways, too. Sirius was louder, livelier, more interesting. He had his gaggle of friends that hung onto his every words and crowds of girls that practically drooled over him. Disgusting.
Everyone knew when Sirius walked into a room. He waved his hands when he talked, forcing you to duck out of the way to avoid being smacked around the face. He delivered ridiculous speeches while standing on tables. He made outrageous swerves and dives on his broomstick and lapped up the applause.
He was just big.
"Come on boys, let's get you measured up."
"Aunt Cass, aren't we getting a bit old for that?" Sirius grumbled.
"Nonsense! You're fine, strapping young men and you're still growing and you will be measured on my growth chart. Regulus, you first."
Regulus rose from his armchair and stood obligingly in front of the wall where all the Blacks had been measured up for the past three generations. Aunt Cass's quill flew up to mark a line at the top of his head.
"Two inches! My my, you'll be as tall as your father yet, Regulus," Cassiopeia beamed.
Regulus straightened his back, a little shot of pride bursting through his veins, and sat back down.
Sirius was called up next, rolling his eyes but walking with a swagger all the same. Regulus pretended to inspect the cuffs of his robes but was really keeping an eye on those height marks labelled Sirius. Frustratingly, they jumped up even higher than his had.
"Maybe next year, baby brother," Sirius winked, and clapped him on the shoulder.
Maybe next year.