I think the latter, considering the games were heavily inspired by Battlefield it would make sense to prioritize multiplayer first. I think it also makes sense when looking at the objectives in the campaign, which mostly (if I remember correctly) are really different multiplayer objectives: Kill a certain amount of enemies, capture this command post, grab the intel and bring it to the "base" etc.
I've heard that apparently the campaign in BFII was added very late in the development, I don't know if this is true but if so that would mean the maps were designed for multiplayer first.
Another thing that came to mind as I typed this is the fact that all the maps are the exact same ones regardless if you play the campaign or multiplayer/Instant Action. In say Call of Duty 4, you can tell the multiplayer maps are heavily inspired by locations from the campaign, but rarely do they look exactly like their campaign counterpart.
Of course, I don't know how well that argument holds up considering that the campaign in BFII is supposed to capture a similar feel as the rest of the game by being part of large battles, so using the same maps designed for large scale battles makes sense. Or it could be just to save a lot of time, since the game came out a year after the first one after all.