Ja Rule -- I'm Real ... HUUUUGE!

Apparently Ja Rule smuggled a couple of guns out of prison ... 'cause the dude is YOKED!!!!

The rapper just posted a photo to his Instagram page ... showing off the jacked physique he built while serving out his 2-year sentence in a NY prison for attempted illegal gun possession and tax evasion.

The 37-year-old rapper was obviously hittin' the weights during his time in the big house ... and clearly the hard work paid off ...

Unlike some other prisoners we know ...