Maybe things are not so exponential (is this the term ?).
If we analize the differences with a metric criteria, we may find they are not so great in proportion terms.
Starting by double checking the weight of each example; mine weighs precisely 4,500 Kgs ... so a little more than originaly quoted. Is the weight of Jean-Luc's example a straight one or is it (also) passive of a double check ?
Then,
and above all, we have the shape of each breast plate; mine has a much lower cut on the neckline and armpits. A lot of material less = a lot of weight less.
Concerning strenght and speaking of laminated construction, as tought by Allan; my example may not have multi disperse drops, but surely has a whole double layer.
Finaly the shot proof detail; not discussing the strength of the bullets used by different smiths
, i would swear the dimple in the middle of my example is (also) the reflex of a shot.
.