The theory, in short: The Muppet Movie is a true-ish origins story about how the Muppets became actors; all Muppet films and projects are the works these performers have created under Lew Lord's Standard Rich & Famous Contract, and that's why each movie (and the tv shows) can vary in terms of Kermit and Miss Piggy's relationship, their relative level of success, etc. The Muppets, in turn, is the movie about what happened to that troupe after decades in the limelight, after starring in those films and having all those famous people show up on "The Muppet Show."
Two excerpts suffice. First, on dealing with the success of The Muppets:
Well, we hadn’t done a big movie in a long, long time, so really just seeing the guys everyday was celebration enough. Because, you know, when we’re not working, we go and do our separate things. We don’t live in a big house or anything. I don’t really spend a lot of time in Hollywood. I go back to the swamp. Piggy usually won’t come to the swamp with me—she just goes off with my credit card and I don’t see her for months. It’s very strange.More tellingly, as to a character in the next movie, Constantine, being his doppelgänger:
We talked about putting me in makeup and having me play both roles, but we decided Constantine needed to be a guy who could do a Russian accent. And, you know, I’m a pretty accomplished actor and all, but besides the Muppet Christmas Carol and Muppet Treasure Island films, I’ve only really ever played myself.HT: @sepinwall, @suddenlyissoon