but why? Unless you are talking about a badly balanced mod that forces you to travel around the world, which is beyond the point, as that is not about the "size" of the map (read number of regions/towns/castles available + travel time/speed).
Lets say you have Europe as the map. You can spend your campaign inside England, doing some attacks/raids around France and Ireland/Scotland. You dont need to go to Spain to do quests for lords there, or Italy, or Poland ... those regions would be there for future campaigns or as places you can lead the royal army for wars, or places you can lead your warband for raiding before going back home (viking style
)
As mentioned earlier and also above its about the design to make the campaign not a grind experience.
Of course if you define a fun end game as a map where you can conquer 100%, then yes, having a massive amount of places to conquer would be a drag and and you should avoid those kind of mods.