It IS one big map.
Missions (and campaigns) are limited to the area that gets loaded at the start of the mission. In that way, each mission is "stand alone". Meaning, you can't drive from one mission into another one. Each mission must be loaded on its own.
I've run off the map on some test missions (in DW and SC) to see what would happen and you just cover more ground. Besides the sea bed and land masses, there's nothing to interact with.
How that gets translated in the different games
may be different.
