It's not all palaces and couture at Kim's wedding!

All is not quiet on the Kardashian front, as X17online can report exclusively, Rob Kardashian has left Europe to return to Los Angeles AHEAD of the wedding, after a fight with Kim!!!

And you'll never guess what the fight was about ... Rob's weight!

X17online had the EXCLUSIVE pix of Rob departing for Paris with Kris last Sunday and the shots drew 100% of the media attention AWAY from Kim and Kanye. Could that have been Kim's beef with Rob's beefy frame?

A source tells X17online, Kim and Rob had it out Friday before the lunch at friend/designer Valentino's chateau. Kim wanted the departure from Paris to look like a catwalk, with every member of the Kardashian clan wearing beautiful Valentino couture and looking like a million bucks. Obviously Rob didn't fit into that plan and that's when the war began ...

    "Rob said he was sickened by Kim and Kanye's display of wealth and clothes and beauty and called it 'superficial bullsh*t' but Kim was pissed that Rob hadn't made an effort to drop any weight over the past few months. Rob basically thinks Kim cares about him only for his appearance and he said he'd 'make life easier' for her and just leave."

Whoa! Talk about wedding drama ... apparently when the cameras are rolling 24/7, the pressure to look good can get to a family. In fact, the timing of Rob's supposed fat camp retreat coincides with the amount of time he'd need to drop is excess weight before the wedding. And it doesn't seem coincidental that we saw momager Kris Jenner escorting Rob to his reported health treatment center. She wanted Rob to be picture-perfect by mid-May ...

Our source goes on to tell us:

    "Rob booked his flight for Saturday morning and skipped the lunch at Valentino's. He just doesn't seem to feel comfortable or happy around his family at all. They're all wrapped up in the same pretty world and he just doesn't fit in."