Joe Jonas in the Shred Zone

Joe Jonas is flaunting his body of work ... 16 weeks of intense work to be exact.

The DNCE frontman bulked up and then shredded by weight training and boxing at Unbreakable Gym in West Hollywood. The gym's co-founder (and 'NFL On FOX' star) Jay Glazer says Joe committed to a pro-athlete training regimen that included specific caloric intake and two-a-day workouts.

Can't argue with Joe's results, but here's the thing -- his little bro, Nick, pumps up at the same gym, and recently showed off a bulkier physique.

So, we gotta ask ...