The Game: My Junk Is Massive for the Underwear Biz

The Game's sizable manhood made a huge indent on underwear sales ... TMZ has learned.

Amazingly, Game's package wasn't the only thing people noticed in his IG posts seen 'round the world -- because a spokesperson for Ethika tells us sales have erupted more than 500% for the 2 styles the rapper sported.

We're told Game doesn't have a deal with the skivvies co., but they did send him some swag in August. With the huge sales boost ... Game's gonna be so vein.

#PowerOfthePenis