What is the difference between a quay and a wharf?

A "wharf" is "an area like a wide wall built near the edge of the sea or a river where ships can be tied and goods can be taken off them," says the Cambridge Online Dictionary.

A "quay" is "a long structure, usually built of stone, where boats can be tied up to take on and off their goods," says the same dictionary.

Does size matter here? I mean, ships for wharves, and boats for quays? Because, apart from that, I can see no real difference.

Solution 1:

The Merriam-Webster Unabridged definition of wharf (probably paywalled) contains a Synonym Discussion that notes (emphases and formatting mine):

• wharf, probably the oldest of the terms applies to any structure projecting from the shore
• quay normally applies to wharves or piers characteristic of small places
• a quay is a docking facility at which ships lie parallel to the shoreline

So: a quay is a wharf, but one constructed in a small place, in such a way that ships docked at it lie parallel to the shore.

Solution 2:

"Quay" and "wharf" are originally different in usage only really. A "wharf" is for the commercial unloading and loading of ships and is (usually) private property belonging to a shipping company. A "quay" is for smaller ships or boats and is available for general use, these are frequently owned and operated by the local governments/councils for the benefit of local fishermen originally. Passenger ships would usually dock at "quays", cargo ships at "wharves".

A "wharf" -

A "quay" -