If Chouji is looking for it, the passage of time is written more obviously across him. He's aged; a little taller, a little more muscle, and features sharper or at least less-rounded than they were before. He's gotta be nineteen physically, by now, even if he doesn't count it until his birthday.

And Shikamaru can tell, just from the joke's existence and not necessarily how it was told, that it bothers Chouji. He reaches out to grab his friend by the shoulders again.

"So troublesome." A grin. "Don't worry, though. For you it's worth it."