What I have deduced from being a minour Pop Culturologist and a huge Pixar fan is this: All Pixar films (With two exceptions) take place in the same Universe, but different time periods and... dimensions. Brave, I don't really count because it's so radically different and doesn't contain the stuff that binds the other films together, so lets just call that where it is: Medieval Scotland. The Toy Story movies, A Bug's Life, Finding Nemo, Ratatouille, and Up all take place in the same universe around the same time. Wall-E is the last in the timeline being 700 years in the future and Earth is abandoned. The Toy Story movies take place in real time due to the maturation of the characters. Toy Story 3 takes place a little while after Up (Not the intro, obviously) because Andy's wall has a postcard from Carl and Ellie Fredrickson on it. Also, Lotso appears in Up, and Rex appears in Wall-E on his rotating shelves. A Bug's Life and Ratatouille can take place basically anytime between these movies, even the years they came out. (1998 and 2007). Finding Nemo can also share this distinction, but just with the addition of taking place in the Ocean. What about Monsters Inc? Different dimensions obviously. It's in the same universe, but in the Monster dimension. Randall becomes trapped in the mobile home that was shown in a Bug's Life, and Jessie, the Luxo Ball, and a Nemo doll are all in Boo's room. This could mean that Boo is really Emily, Jessie's owner, but that's been debated. The Incredibles? It doesn't physically exist, but it's a franchise in their world. In Nemo, there's a kid reading a Mr. Incredible comic in P. Sherman's office. This means that The Incredibles is a huge multimedia franchise (Their Avengers) and we just happened to see one of them. And Cars? Cars doesn't count. They're all Cars. Screw Cars.
Huh alright, though I won't mind it staying in Yack Fest, since again this is about a silly fan theory.
CG for short