Here’s a write-up based on the phrase — which roughly translates to “My little brother is seriously huge, but it doesn’t sink in / I can’t wrap my head around it.” This is interpreted as a humorous, affectionate, or slightly bewildered reflection on a younger brother who has grown physically large (tall, broad, or imposing), yet emotionally or perceptually still seems like the “little” brother.
"Well, tell him to bring his head."