Why was Darth Vader so much taller than Anakin Skywalker?

In A New Hope, Darth Vader is seen as extremely tall. He is perhaps close to 7' tall, although other sources cite his height closer to 6'8.

In episodes 2 and 3, we see Anakin is the height of a normal sized man. Why would he be so much taller as Darth Vader?