Blac Chyna to Rob: Back of the Air Bus, Bitch!!!

Good thing Rob Kardashian's lost all that weight, because his knees are about to make contact with his chin.

We found out Rob's flying economy this weekend ... where he can barely look through the curtain and see his fiancee in first.

TMZ got hold of the rider for the couple's appearance Saturday in Vegas ... at the Tropicana's Sky Beach Club. They will get 2 round-trip plane tickets ... one first and one economy. And we know who's not in first.

Their rider also calls for multiple bottles of booze ... bad idea for both.