What he said. From Wikipedia :

While it looks heavy, a full plate armour set could be as light as only 20 kg (45 pounds) if well made of tempered steel. This is less than the weight of modern combat gear of an infantry soldier, and the weight is better distributed. The weight was so well spread over the body that a fit man could run, or jump into his saddle. Modern re-enactment activity has proven it is even possible to swim in armour.It is possible for a fit and trained man in armour to run after and catch an unarmoured archer. The notion it was necessary to lift a fully armed knight onto his horse with the help of pulleys is a myth originating in Mark Twain's "A Connecticut Yankee in King Arthur's Court", and only rarely occurred in the 19th century. Even knights in enormously heavy jousting armour were not winched onto their horses. This type of "sporting" armour was meant only for ceremonial lancing matches and the design had to be extremely thick to prevent severe accidents, such as the one causing the death of King Henry II of France.
Granted, the suits of armour worn by re-enactors are made out of better quality metal, and are probably a bit lighter than the real thing because of this, but on the other hand your run-of-the-mill re-enactor is a librarian, historian, scientist and so on, not a fit, trained and exercized military man.

This idea that plate armour was super-heavy stems from a complete misunderstanding of the whole point of platemail : the idea was not to take the hits like a WW2 tank armour would, but to deflect them harmlessly, mostly through the use of curved surfaces and clever design. So they didn't need to be that thick.

(To be 100% accurate, WW2 tank armour was often sloped, and thus also caused shots to glance off sometimes but it wasn't the reason it was sloped in the first place - it was so you had more "effective" thickness compared to a vertical plate, as the incoming shell would have to punch through more metal. So in essence, it was the brute force approach to armoring )