The world's smallest violin
To be sure, there are any number of actual, real very tiny violins, playable or not, that may vie for this title. But this entry is not for any existing violin, but only for a metaphorical one. The "world's smallest violin" is a phrase used in a sarcastic manner to dismiss someone else's troubles as imagined or very minor. Typical use would be accompanied by a hand gesture of rubbing the tips of the thumb and forefinger together in tiny circles and asking "Do you know what this is?" And then replying that's it's "The world's smallest violin and it's playing just for you!" Sometimes, a real or invented song title is inserted, depending on the user's creativity.
The metaphor has successfully transitioned from oral folklore to Internet memes.
The metaphor assumes deep cultural familiarity with non-diegetic music - film/TV scoring. But enhancing sad scenes with violin music surely predates the existence of television and film.
The origins are unknown at best.
Author Stephen King used the the same variant of this metaphor in two of his works, 1982's Different Seasons:
I rubbed my first finger against my thumb. "This is the world's smallest violin playing 'My Heart Pumps Purple Piss For You,'"
And much later in the 1997 novel The Dark Tower IV: Wizard and Glass
Eddie held up his right hand. He rubbed the thumb and forefinger together.
"WHAT DOES THAT SIGNIFY, FOOLISH CREATURE?"
"It's the world's smallest violin, playing 'My Heart Pumps Purple Piss For You,'" Eddie said. Jake fell into an uncontrollable fit of laughter.