Kim Kardashian -- Post-Baby ASS SELFIE ... It's Big & It's Back

Kim Kardashian's gargantuan ass is back in working order ... and last night, she let the world know ... with a backside selfie on Instagram

It's the first butt-selfie Kim's taken since giving birth to Kanye West's baby North back in June.

Kim's been on a serious sexuality kick lately -- on the most recent episode of her reality show, she talked about wanting to pose NAKED asap ... saying, "As soon as I get in shape, the first thing I want to do is Playboy or some nude shoot. I just wanna walk down the street fully naked."

Looks ready to us.