Anyone who's ever worn actual chainmail can attest that it is more cumbersome than a well designed suit of plate. The weight of all those links, unless made with modern alloys, is substantial, and as most of the weight is on the shoulders, it can wear you out quickly. Add in the heavy gambeson worn underneath, and it becomes even more bulky. A good waist belt helps to alleviate some of the burden, but not by much. Chain armor's attraction was that it was relatively cheaper, and quicker to produce. I hate it; wore it a few times, and quickly went to leather and plate.
Good plate armor will see the weight distributed much better, and late period plate armor was typically lighter than equivalent chainmail, but as each piece must be individually fitted, the construction process was much more detailed, as well as expensive.
Check out this match between the Baltic and Israeli teams at last week's epic Battle of the Nations to watch just how well a big man in full plate can maneuver, action starts about midway.