Why do some cruiseships dock in Darling Harbour and some in Circular Quay?

Question

I noticed there was a really big cruise ship in Circular Quay in Sydney yesterday however sometimes I notice they're in Darling Harbour. What's the difference between docking in the different spots? I thought that some may not be able to fit under the Harbour Bridge but why not have all at Circular Quay then?

Answer 1

circular quay is deeper water.

i have never seen big cruise liners in Darling Harbour myself.

they wouldn't fit under Pyrmont bridge anyway.

Answer 2

Some can't fit.  

There is the Overseas Passenger Terminal at Circular Quay, where they do all the overseas crusises, I suppose.

It's like having 2 airports.....

Answer 3

Size, it is all about size.