But is there really a causal link between game length and quality of content? other than exactly in the other direction (i.e. the longer a game is the more likely it contains large sections of filler content suckitude).
I'd argue it's significantly easier to create a game with 10 hours of top notch quality, polished content than to create one with 50 hours.
If most "good" games are of 30-60 h length it's because most games are of that length, naturally the good ones among them will also be of that length. Implying it's the length that's decisive for the quality of the content (longer = better content, short= content must suck) is a fallacy.
You're exagerrating a lot. First it's not exactly "short = content must suck", it is that maybe your game is short because a longer game wouldn't support your system, in which case although you were right no to try and bloat the length, the fact is that your system is still just too simple for your game to be some top tier one, or at the opposite you just didn't take the time to fill your game with content supporting your systems, once again it's alright but you could have done better.
Different genres are differents. In Super Meat Boy one more level is just one more level, you can add or remove a level (the probable stupidity regarding difficulty curve put aside, you can even technically totally randomize the order of the levels) and it doesn't change the rest of the game, and for that reasons you can make a 30 minute very good plateformer. For a more organic overall experience an adventure game like Day of the tentacle mostly consists on one big section, and I'd say you can't make a much shorter top tier adventure game (less that, let's say, something like 4 hours) because to be at that level of good an adventure game requires at least such a big section of interconnected puzzles.
And RPGs are of one more kind of beasts, you can't distinguish different sections of an RPG like you would distinguish different levels in a plateformer or sections in a adventure game because managing your party through the entire game is half of the experience, seeing your party slowly growing stronger and learning how to use each member effectively, unlocking new skills and spells, ect. If the RPG system is not complex then it's not good, and if it's complex then the game needs some meat where using all those new options (spells, skills, weapons) you unlock during the game or the game is not complex overall so it's not good.
How good are we talking about? Sure Paper Sorcerer is pure fun, Voidspire Tactics is also pure fun, the game has perfect system and content for a 20 hour game which is exactly the length of the game. Swords and Sorcery : Underworld is a very nice little Might & Magic experience, but compared with Might & Magic 2 I've played recently it just have much less content, it's not shorter for no reason, it's shorter because it's overall a simpler game. I'm very glad they made Swords and Sorcery : Underworld that way, they're not a big team and there's absolutely no reason to try and make your game longer than its content by tricks, it's just stupid, but ultimately putting the two game one near another and saying they're just as good is a bit of an exagerration, like saying the awesomeness of Worlds of Xeen has absolutely nothing to do with its total length. Naturally long games are great.
Feel free to disagree but I consider that Paper Sorcerer, Voidspire Tactics or Swords and Sorcery : Underworld are worse than the very well-made long-ass non-linear complex big party games with tons of content, with a harsh, slow beginning, where you slowly have to improve by finding some solvable areas like Disciples of Steel or whatever, there are not tons of such very good very big games but they compete in another category as far as I'm concerned. It's definitely harder to make a good long RPG, and if you can't for whatever reason then just don't, just make a to-the-point short game, but there's much merit in managing to doing the naturally long one. It's not about having a 40 hour game with 3 actually different and interesting skills, 3 actually different and interesting enemies and 3 times the same dungeon (well by the way overall the dungeon layouts in Disciples of Steel actually are very far from impressively good but anyway) to make your game longer, which is indeed utterly stupid, it's taking the time to implement all kind of different skills and spells and enough unique situations so that composing and managing a 8-character party is interesting, and then different dungeons, situations, enemies where you'll get to use them for on overall big satisfying experience, otherwise what you have made is just some dumbbed-down or simply rushed game, which may be very fun but can't compete with ambitious games which fullfill their ambitions such as for example Star Trail or several Might & Magic games.