Players want to unlock content and the most efficient way to do so is to maximize how many FPS games control progression speed: SPM or score per minute. Score is usually a formula composed of objectives and kills. The key is that it’s uncapped: there’s not a fixed amount of XP up for grabs in a given match or time played (this would be a better design). The formula implies that the more “action” in a given minute of gameplay then the more score per given unit of time and the faster a player will progress. Small maps excel at encouraging this – there’s a short amount of time before you bump into an enemy or objective.
FPS players like small maps because they function as costless XP boosts. Nuketown will be making its 5th appearance in the CoD title with Cold War.