Britney Spears' Ex-Fiance Jason Trawick -- Gutsy Move

Freedom isn't the only thing Jason Trawick gained after breaking up with Britney Spears -- and he's not tripping one bit ... proudly showing off his new gut during a weekend pizza run in Hollywood.

Oh yeah, he got Popeye's too.

Spears and Trawick -- her former agent -- famously started dating in 2010 and were engaged the following year. Barely a year after that, they were single again.

What a waist.

As for the pizza ... our own Gary says, "All fat guys know ... you never eat the pizza out of the box right outside the store. C'mon man."

4:28 PM PT -- Jason just responded on Twitter with a big belly photo, writing, "Thanks @tmz I needed this. Started my diet/workout program today. C u in a month!"

You're welcome ... but a month's pretty ambitious.