anyone can help explain why expansion lowers morale

I think this is solely for balancing purposes and this will prevent nations from suddenly taking over the world.

Ok. If you want have full answer, why expansions lowering moral, you should look at human history. Every single empire exept GB (they went by kind of violeteless uprising) and SU (wich selfmurdered by own citizens) died after oversized expansion.

Government expences rises over proportional to encome. Defending expences too. Mood of citizens falling cause of lowering life standards.

It is very short answer, but it should be enough.