Knights aren't modern style soldiers, there is little to no standarization in this regard.
For starters, for a given fitness level weight and height are connected.
According to
this, being able to get onto a warhorse in armor was considered a pretty basic expectation, so that gives us a bit of an idea.
So overall, a good knight would have similar BMI to what's considered fit today, but a lot of the ones not really training for warfare as they are supposed to may be a bit fat, especially if they had the wealth and/or safety to get away with it for previous years - though i guess after a few years of constant campaigning those cases may be trimmed down.
So overall, their weight would probably be equivalent to medium-heavy weight boxers, special forces soldiers, and equivalent athletes for the ideal, somewhat above that for those with softer, more comfortable life, and below that in cases like long term supply shortages in campaigning, or as in the given example of Templars, excessive fasting.