Like a lot of us as we age, movies tend to get a little bit fat.

Consider some of movies that were so influential or successful or hit upon such a marketable hook they got inevitable sequels and, in many cases, entire franchises. Star Wars. The Matrix. The Fast and the Furious. Alien. The Terminator.

What do they have in common? They were all kind of small. Beautifully simple. Nimble and zippy. You could encapsulate the entire premise in a single short sentence.

A farm kid longing for adventure joins a princess and a ne'er-do-well as they fight against an evil empire. A robot is sent back in time to kill the mother of humanity's future saviour. A hacker discovers the world he knows is a simulation and joins the fight to liberate humanity. Alien... about, you know\... an alien.

And what did they all end up as? An entire galaxy's worth of mythology and lore. A war movie so full of it own profundity all the cool evaporated. The seeding of the human race, precipitated by an ancient life form in cahoots with the one of humanity's robotic servants.

A nuclear apocalypse that happened, then didn't, then did, then John and Sarah Connor mattered, then they didn't, then part of the marketing of the entire franchise became (descended into, more like) 'ignore everything that came after the last two films, they're no longer canon'.

You might wonder why I'm including The Fast and the Furious on the above list, by the way. It's by no means a modern classic. But it's the most egregious and hilarious example. What started out as a gang of illegal street racers robbing DVDs from trucks in Los Angeles has become such an orgy of globetrotting stunt action (to say nothing of the constantly ballooning cast) James Bond writers would tut and say 'well now that's a bit of a stretch'.

It's one of the reasons behind a very common phenomenon in filmed entertainment, the one where sequels to films are seldom as good as their predecessors.

For every The Godfather Part II there are a hundred examples of Wonder Woman 1984. For every The Dark Knight there are a thousand films like of Jurassic World: Dominion. For every Aliens (yes, it's an example of a sequel that in many ways surpassed the original before it devolved into absurdity) there are ten thousand more like Highlander 2: The Quickening.

But while we're all painfully aware that it happens, can we discern why? Is it because writers run out of ideas? Studio suits shave off all the rough edges and make it too corporate after becoming too involved when it becomes an important property?

Maybe, but there are legitimate arguments against such claims. I don't mean to pick on poor George Lucas again, who comes in for brickbats every time I talk about writing on this blog, but he never ran out of ideas; he long claimed he always intended to make three trilogies of his galactic civil war space fantasy.

And he certainly didn't suffer studio interference, financing Star Wars Episode I - The Phantom Menace himself.

The first problem as I see it is that when we (the audience) make a movie a success, everyone from writers to the studio greenlight committees see it as a signal that we want to know everything about the world depicted. Even we as moviegoers make sometimes the mistake of thinking we want that.

But the premise of a war with sentient machines keeping us enslaved in a simulated reality to power their civilisation was never the point of The Matrix, is was just the scaffold upon which the characters got to do some very cool shit once they knew how they bend 'reality' to their will.

The whole backdrop of the war with the machines was something we'd seen before (in fact in another film on the above list) and it wasn't nearly as interesting, but the Wachowskis took our interest in the simulated world as a sign we were just as interested in the real (outside the simulation) struggle as well, and what had been a very fleet-footed action thriller turned into an overblown war movie, drowned in its own self-importance.

In response to us loving the characters, swashbuckling romance and adventure in the Star Wars universe, successive creators and even studios have mistaken that for a belief that we consider everything in that universe to be dreadfully important, and that we're interested in every element of the mythology, cultures, systems and characters in it.

Star Wars isn't an altogether fair example because plenty a lot of fans are actually interested in every planet and species. We were all ready to lap up anything even remotely connected to the franchise, and I was one of millions glued to our TVs for 1978's The Star Wars Holiday Special despite how awful it is.

But you also can't deny plenty of series and movies have fallen flat because they're about places and characters we either haven't cared about or which haven't been done very well. That self-importance that usually emerges does so when they think we love the world and everything in it rather than the worldbuilding.

A creature that looks like HR Giger's repressed Freudian nightmare stalking a group of roughneck delivery drivers aboard a cavernous space station is a story in a world, but that story is all we needed to know to enjoy Alien.

The world is where that blue-white CGI guy with the scowl (The Engineer, I think he was called) prompts the evolution of the human species, and that somehow the species that becomes the alien and facehugger is wrapped up in it.

But that's a world, not a story. Does watching a scary monster creep up on a small group of people, picking them off one at a time in the dark, mean we're interested in the world?

Consider Mad Max and then Mad Max 2. Both are action movies about a cop who drives, a ronin figure who rights wrongs in a post-apocalyptic world. Mad Max 2 doesn't try to explain the world, it doesn't need explaining. It's just Max doing what he does best, with bigger, better, more colourful characters.

George Miller did it again in Mad Max: Fury Road decades later with a new Max and a new mission, but it didn't try to plumb the depths of the political and economic reality of Max's world. It showed them, but other than that it didn't forget it was about Max, insane driving action and larger than life characters even larger than last time.

Or look at the Saw films. Love them or hate them, I'll defend the writing in them to my dying breath. They're always only about John Kramer (Tobin Bell), his traps and the awful people he captures to put in them.

It builds the mythology behind John out; every subsequent film reveals how someone we thought was a victim was actually an accomplice, and before it's over the guy we thought was a lone operator had an empire of helpers and plans that put him ten steps ahead of everyone. But it's about his traps, his accomplices and how he's always ten steps ahead of everyone. It's about the characters (however thinly drawn; I'm not suggesting Saw is Shakespeare).

By contrast, most sequels get increasingly bloated because they're not about characters but about the lore. The first film might depict an interesting world, then the increasingly convoluted sequels usually develop along the lines of presenting more information about the world, then some hidden history of the world, then how everything you thought you knew about the world was wrong, etc etc.

Maybe it happens that way almost by natural attrition because for all the talk about how movies and TV are only really about characters, it's something Hollywood is terrible at. It's great at thinly veiled archetypes, which might be why writers feel they have to introduce the larger environment and all the machinations that drive it, thinking we'll be just as interested in the set dressing behind the people. If they stay with just characters they quickly run out of things to talk about.

What franchises do you love that moves fast and breaks things but have retained their creative souls instead of morphing into mere studio-engineered product?

On screens recently, only two movies have really grabbed me. One is Michael , which for all its flaws (and there are plenty) it gets one thing right; the buildup to the mic drop many of Jackson's songs contained, making you look forward to the breaking out of another of his greatest hits.

I was also interested in The Sign of the Cross, an example of pre Hays Code filmmaking where director Cecil B De Mille actually gets away with some stuff that would have the Christian Right foaming at the mouth just a few years later.

Along with the rest of the developed world I also duly went along to The Odyssey. I was thoroughly underwhelmed with Oppenheimer and after the overstuffed confusion of Tenet it's been a long while since Chris Nolan really impressed me. It's better than recent efforts, but it's still not a patch on Inception, Memento or even The Dark Knight.