UFC's Nate Diaz Parties with Snoop AND BIGGEST JOINT EVER!

The Diaz brothers were playing with fire Sunday night -- hanging backstage at a Snoop concert ... with Nate holding (what appears to be) one of the BIGGEST JOINTS EVER!

Nate Diaz and Nick Diaz made the 40 mile trek from Stockton to Concord, CA for Snoop's show -- where they also kicked it with Kurupt and famed Bay Area rapper Berner.

No word if Nate actually smoked the joint -- or if it actually contained marijuana -- but it's a troubling image considering Nick was just reinstated following an 18 month ban after testing positive for weed.

Hey, maybe it's just a prop ... right???