"Too much content" definitely does sound like a bit of a misnomer; and depending on how you look at it, the phrase really is a misnomer. Many of the cases here can actually be explained as "poor pacing". Hypothetically speaking, a game can be as long as it wants (though I would at least like the thing to have a definite ending) provided it remains engaging throughout.
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To start off with the obvious, we have the sandbox RPGs made by Bethedsa -- The Elder Scrolls series in particular. The pacing in these games might as well be non-existent, their purpose is to give the player a large world to play around in and explore. This leads to the player defining their own pace, which can be anything from "make a beeline to the finish of the central storylines" to "run around in circles". They also have what could be called "unnecessary content", but the parts which one finds worthwhile varies from player to player (if not playthrough to playthrough). What some view as an unnecessary quest can be vital to others. It's fairly safe to assume that the developpers aren't expecting most players to do everything there is in the game... though that doesn't stop some from trying.
MMORPGs and their persistent worlds aren't much different than sandboxes, only that the "pacing" and "storyline" are even less important. Again, the player defines their own pace; however, the community tends to gravitate towards "beeline to the level cap and the best gear available". While some games do award those who try to complete all the content in the game (for example, the "Loremaster" achievement in World of Warcraft), it's generally expected that most will not bother with that; what is expected is that they'll endlessly repeat doing the newest content for the best stuff.
Anyhow, both catagories could also be called "endless games". These cram in as much content as possible, regardless of if it's necessary or not. "Too much" is a bit hard to apply when the only limitation is the player's interest in it (even if most are unlikely to stay all the way through).
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In more story driven games, pacing is present rather than non-existent. However, longer games do have a tendency to have bad pacing; this is arguably very apparent when the player is given the choice of which order they want to do the objectives. In other words, Bioware's games generally have poor pacing. Dragon Age: Origins is a very lore-intensive game, but the sheer size of the adventure makes it exhausting. If you take the time to do every single sidequest, it can feel even more arduous. Dragon Age II has even worse pacing, thanks to it's unfocused plot; this is made even worse with the repetitive encounter design (waves! waves! WAVES!) and excessively recycled environments, which are the two inexcusable issues with the game. Both games have loads of "unnecessary content", but they can be trimmed down a fair bit if you want to; compulsive completionists might run into burn-out issues, however. Mass Effect (1)? Exploring worlds in the Mako for resources and other trinkets are almost certainly "unnecessary content", but the central storyline is fairly tight & cohesive; it only feels like "too much content" if you're trying to do all the sidequests. ME2 has better pacing than the two games in the Dragon Age series, but not quite as good as ME1's central storyline; the sidequests feel less extraneous, at least.
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Then we have games which are a bit more focused in their story (and have a slight tendency towards linearity), but still come out to a sizable length. The first group is the Final Fantasy games (particularly looking at those from previous generations, the recent batch hasn't been quite so good) tend to be very long games, but their pacing is good enough that they don't feel too long even after 40-50+ hours. They're just an epic adventure (in the classical sense), long journeys and remain engaging throughout. Even if they do open up towards their conclusion with a "last-minute-sidequest-binge", the sizable adventures whisk you from one event to the next without ever having the player loose interest. Final Fantasy IX stands out for me, with a near-perfect pacing of peaks & valleys constantly inching the player forward through the storyline.
Another group is the almost universally acclaimed Legend of Zelda franchise, and strong pacing helps the games along immensely. Interestingly, most games these days start off with a big band... but not the LoZ games, which begin with a fairly lax pace to ease you into the adventure. Most of the games are a fair bit shorter than the others mentioned here, but Twilight Princess actually gives them all a run for their money at 25-50 hours; that's comparable to both Dragon Age titles and many Final Fantasy titles. For some reason, some people felt it was too short despite being longest LoZ title by a longshot (pun not intended); so unlike many of the examples of games having "too much content", it didn't feel too long nor exhausting despite having a similar amount of content. And that is the power of good pacing.... or maybe people were complaining for the sake of complaining (as usual). A recent bit of news had Nintendo say they felt Twilight Princess was too unfocused, something which they've corrected in Skyward Sword; how that turns out remains to be seen.
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To look at the other end of the spectrum, there's the recent batch of short action games. First person shooters and hack-&-slash games usually run between 7-10 hours, with the odd one going up to fifteen or so. The pacing in these games tends to be stellar, but at the same time you're more or less on the rails. No chances for exploring or that. While these games certainly don't have "too much content", they're trimmed to their essentials; much of their "longevity" comes from great replayability and/or multiplayer.
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Anyhow, I think the main issue isn't so much "too much content" as it is a mix of poor pacing (if any at all, in some cases) and "unnecessary content". Done right, a game can be as long as it wants. Easier said than done, of course; keep good pacing & making all the content feel necessary to the whole becomes exponentially more difficult as the game gets longer.
Still, some games have managed to do it right.
Oddly enough, one of the best examples of such is NOT an RPG.