Justin Bieber -- Retraction Victory ... He's Hunky and He's Packing

It only took a matter of hours before the website that posted a GIF making it appear Calvin Klein artificially endowed Justin Bieber with muscles and genitalia folded like a cheap suit.

TMZ broke the story ... Justin's legal team fired off a threatening letter to BreatheHeavy.com, demanding a retraction after the site posted a GIF making it appear that Bieber was way scrawnier and "smaller" than the Calvin Klein photo would suggest.

BreatheHeavy just caved and you can literally feel the fear. The site says, "Bieber denies the photo is real, and I respect that and will believe him."

But the site didn't stop there, quoting Bieber's personal trainer ... who said, "I can definitely confirm that he is a well-endowed guy. I sound weird saying that, but yes.”