To better fit the royal egos through.

Ha! I kid, I kid.

Most likely, it was mess with people psychologically, adds to a better defense and simply, it looks more impressive than if you were to have a ordinary sized double doorway.

Psychologically because, are you more likely to want to go through a massive double door entryway or a normal sized single doorway? There's wonderment and/or worriment as to what lies on the other side of these doors than others.