I believe that a balance between both quantity and quality is necessary to make something great. Prime examples of games that get this balance right are open world games such as any Grand Theft Auto title, The Witcher titles etc. The main stories of these of these take dozens and dozens of hours to complete, but there is a literal shit-ton of side quests to complete, and the majority of side quests sit alongside the main story in terms of quality. Additionally, I am in agreeance with Garpocalypse. Fighting games are the worst genre to use as an argument for the piece (I believe so anyway), because in terms of content, they are generally all the same. Story/Career Mode, a couple of VS. Mode's, Practice Mode etc. The difference is in the aesthetic. Balance aka 'fairness' is the goal, which in a way generates both more quality and more quantity anyway, as if players feel that every character is balanced or fair, they will automatically have more characters that they will be happy to play and perhaps master, which would mean there is more quality within than another fighting game with half the roster being unplayable because of balance issues.
Sure, I've come across plenty of examples of high quality games with low quantity of content, mainly thanks to my Steam addiction. One extremely short game I will always remember playing is To The Moon. The gameplay within is practically non-existent, a combination of directional arrows and one 'YES' or 'OK' button, making it essentially a visual novel, but it had a beautiful story, the soundtrack was extremely moving in conjunction with the story, and I will always remember it. But, it was short. 3-4 hours to complete, with nothing to do beyond that story. I've never touched it again, because there's no quantity. I probably wouldn't recommend it to anyone unless it was heavily discounted.
I'm not going to persuade someone else to pay say, $20 for a book that has had all of the chapters ripped out minus the first 3. They may be the most amazingly well written chapters of any book in history. But $20? Only if the rest of the book was there. The opposite is true as well. If a game (RPG's are the worst when this happens) has 10,000 hours worth of content, but every single hour of that content is terrible, I'm most likely not going to get past the first couple of hours to even bother discovering if the rest of the content is any better.
Another way I look at the argument is: when people say quantity, do they actually mean quantity? Or are they referring to freedom (or the illusion of it) within the gameplay? I realize I may be splitting hairs (in fact, I know I am), but I think it's an interesting way of looking at it. Obviously, as Brandon stated above, all is subjective. Some people may be excited that a game has heaps of different modes that are all 'good'. Others may not care about any of it except the incredible quality of a multiplayer mode.