As said, there's debate whether the exceptionally large breeds were used for mounted knights occasionally, but the average mounted knight in heavy armor would use a mediumweight war horse (usually called a destrier, not a specific breed but a classification), which was both strong enough to carry the load and agile/athletic enough to keep up well. Favoring strength over speed in a horse eventually gets you diminishing returns as the horse becomes not as able to maneuver or endure; a horse that isn't able to move well in combat doesn't have a live rider on it for long.
IIRC, the usual big warhorse was the ancestors of 'Belgian Blacks' (aka Friesian Horses), which is a relatively heavy-for-height horse usually running 170cm tall and an easy 500kg. Broad-shouldered and very strong, but long-legged and fast/agile.
