In an interview with Gamasutra, DICE, the renowed Battlefield developers, talked a little bit about how they approached building single player and multiplayer.
[Multiplayer is] an iterative process all the time. From day one, we are playtesting the game. Every day there's a new playtest, a new mode, or a new map or something. People give feedback, the multiplayer team goes back in, and they change and update. And that could be, like you said, updating the weapons or ranges — the small nitty-gritty details, these kinds of things. So that happens all the time, just like an iterative feedback loop, almost.
But another example is on the single player side, where it's more like, build the whole thing and then you take a serious look at, "Okay, so I'm on this field right now, I can see troops running down there — how can we make this more spectacular?" or "Is it actually too much going on right now? Should we just remove the troops from there, so you have a quiet moment, so you can actually assess the situation and be a bit more tactical? And then we throw everything we have at you when you get down the hill," or something like that.
So you actually move things around — you change the scripting of the mission, you change where you place your AI, and where you want like, "Oh, the cutscene shouldn't kick in here, let's put it further away," or something. Or even down to, a lot on the polishing on the art side is like, "Oh, we want the sun over there instead, so you get the flare through the trees," or whatever. It might be, "We want more bushes here, more foliage," etcetera.
The fact that they playtest multiplayer from day one isn't surprising–hell, DICE says multiplayer is in their DNA in this same interview. The dedication and level of detail in multiplayer is readily evident to anyone who has played the franchise.
As for single player, well…they say some fans like it, and that's why they'll continue to include it, but I still look forward to the day when they focus all of their efforts toward multi-only. One can dream.