The problem is that quality and "how fun the game" is, is all non-quantifiable.
How do you compare game qualities when different people may have different opinions on those content qualities, where there may or may not be less to compare?
The thing is that you can see the content of a game more easily compared to another game because it's content, you can see how much one game has vs the other.
I'm not using this argument with no singleplayer, no multiplayer and all that.
But I can understand your argument:
Let's take a game where there's both. For me, I can't play CoD anymore. It's just not fun, because the content is basically the same. The quality may still be the same quality as it was in the past but I don't see the difference, just a few cool gadgets and guns, but that's just me. I can also say the same for assassins creed, I myself can't get myself motivated to play the series. There is a lot more content, that I'm sure of, but it doesn't necessarily make them better games that their predecessors.
I'm saying that for some games, whether it's only single player, multiplayer, or both, I feel the some games with more content can make a better game. Yes, I'll be limiting my argument to a few examples, but these examples are from my experiences:
Let's take this recent singleplayer game: Fallout 4. Mods make the game absolutely amazing and in a way, it adds content. But let's forget the mods.
The game itself was pretty fun, but one thing I did not like was the settler system. I wish it was more fleshed out with more content. I felt that, that would've made a better game if that system had more content and was done properly. Sure, I'm including some qualitative changes but at the same time, it felt like the same thing over and over which each settlement. There was a certain pattern to follow to be successful with the system.
Let's take a MMOs for example, which are multiplayer only. I'm not going to enjoy an MMO where there is nothing to do at the end after you've accomplished all the quests to get to that point. Once I finish the story line, what's the point of playing the MMO anymore? That's why content is released that may increase the level cap, add new dungeons and quests. This makes the game better as you can play longer in a game you invested a lot of time in. Else you're stuck repeating the same quests, fighting the same end-level monsters. Sure, some people don't want to play the MMO longer, and they usually leave when the content is sucked dry until there is more.