Thin vs. Doorstopper RPGs
Currently I'm GM-ing a Star Wars Edge of the Empire campaign. It's rulebook clocks in at 448 pages. The first RPG I owned was the D&D Basic Set which was 64 pages long. Now admittedly it only covered three levels but with another 64 pages in the Expert Set you get up to 14 levels. TSR published a number of RPGs that ranged in the 32 to 64 page length - off the top of my head I can think of Star Frontiers, Gamma World, Gangbusters, Marvel Superheroes, Metamorphosis Alpha, D&D Basic/Expert, Top Secret. Sometimes there was a brief 16 page book which would have very basic rules with it or be full of tables. And sometimes there would be a second set that would supplement the first - for example Star Frontiers had Knight Hawks and D&D had Basic and Expert.
From today's perspective, those were some pretty light games. Though at the time it didn't seem like that. I especially played a ton of Marvel Superheroes and Star Frontiers back in middle school.
What's better, the mammoth games of today or those briefer games of the 1970's and 80's? Unsurprisingly, I think the answer is "it depends". Edge of the Empire, for example, provides a ton of options and guidelines. You can make a very customized character with experience set developing very special options for your character - a demolitionist who can shield himself from explosions, a Force-user who excels a disarming foes, etc.
On the other hand, a game like Gangbusters left a lot to player and GM-judgement. For example, while there are the basic details of criminal activity, if a player wanted to build his or her own syndicate then there will be a number of judgement calls to make. If your character becomes known for a certain weapon it's more a function of role play - there typically is no mechanical bonus your character would get.
Both types have challenges - making rapid judgment calls can sometimes backfire on you or go "wrong". But a rules-heavy game can lead to a sort of paralysis while you try to figure out the "right" way to do something.
One thing which did surprise me was when I flipped through Gangbusters there was an awful lot condensed in that 64-page book. You absolutely could run a long campaign with it. Which isn't surprising, considering the number of campaigns run from games like Original D&D or Traveller with just the first three books.