Vanilla Call of Duty? I don’t remember that many people being possible 
For Strategy games, a starting rule in any kind of maps is: Find Resources
After finding resources then you have to gather it
So to get a stable resource line you’ll need to capture/hold the area with resources (directly in games such as company of heroes, indirectly in most other games by building some drop sites and towers/walls/defenses)
In Civ 5, I once play as rome and while the pesky russians hold the area with most iron which i need to build my legions and ballistas (they wont trade me much)
So, i get my troops assault the russian (and some hostile aztec) lands with some help from russian neighbor the Songhai empire
So they surrendered (the Russians had 1 city left, The aztec still have heavily defended area to the north), I got my iron and land (sharing some with the songhai)
As, i went off building my new army to assault the Aztecs, the Songhai attacked me and took over the resource heavy region from me
I remember rage quitting a few more turns later (Rome was practically surrounded)
Everyone wants resources
Big maps: More resources, harder to control
Small maps: Less resources, also hard to control (because the often clash between sides)
I think I would prefer smaller multiplayer maps. But I guess it would depend on the game.