Mine big one is full from the middle up too, but the very lower leaves often fall off or I pull them off when they look sickly.....And I know I don't overwater so I don't get too alarmed, for 10 plus years I've watched my moms drop a lot of lower leaves as it gets taller and taller--hers is huge!
Although you are right, overwatering can def. contribute to them dropping leaves.

That was the first houseplant lesson I learned---not to overwater--that kills more plants than anything else.