Honestly? Not as much as most people think. From about 1000-1300 AD, the standard chain-mail rig in use ran about 60 pounds or so - partly because people, on the average, were smaller back then than they are now. The full-blown plate armor, a la "Excalibur", that came into use around 1400 would have been slightly heavier, but not much more so - here's a modern maker who makes plate armor in the 50-70 pound range. That style armor didn't really last long, anyway - around the time it was introduced, firearms were being introduced also.
All told, a fully-equipped warhorse of the day would have carried not much more than about 300 pounds, which would have been relatively easy for a horse approximately the same size as a modern Clydesdale. The real reason for having such a big horse was because of the way the saddle was set up - when you hit something, the force of impact was transferred from lance to knight to saddle to horse. A bigger horse means a bigger blow to your enemy.