Look at it this way. Horse lords adds a new campaign with a new faction, Raja of India adds an extra map with an added campaign. Conquest of Paradise pretty much adds the native americans as a faction as they were initially a placeholder for hostile ai that you invade for more money. It doesn't feel like a new campaign or faction because in an RTS you would be able to see a new army that blatantly looks like an extra faction. Or it adds a campaign that is labelled as a campaign instead of stuff like horse lords where you get a completely new play style in a new region. If paradox were to limit you at the start it would feel like a full expansion. Imagine if EU4 you started with France, England, Austria and Spain as the nations you can choose. Then they added an expansion that unlocks the Ottomans and Muscovy. Suddenly it feels like an expansion because you get 50% more factions. Instead of having 300 countries you can choose and suddenly 10 get heavily updated. The fact paradox gives you so much at release is why it feels like you don't get as much from an expansion