This system is 32 pounds, which is about the same weight as interceptor with E-Sapi. What's going on?
The majority of the weight in a torso protective system usually comes from the ballistic plates. Everything else is generally kevlar or any other ballistic fabric (and so, the weight from them is usually no different between systems). Discounting the weight from the straps and other 'little' gear, the majority of the weight should again be from the ballistic plates. The choice of material for the plates would affect the weight the most, therefore. Steel plate weighs the most and protects fine if it were HHS (High Hardness Steel), in fact, the best HS (hardened steel) on the market, UHHS (Ultra High Hardness Steel) developed and produced in Sweden, can protect itself from 12.7 x 99 mm rounds with relatively thin plates (IIRC, a 10 mm plate was all that the 12.7 mm round was able to penetrate, and most ballistic plates are about half an inch or 12.7 mm thick, so in theory, a UHHS plate of 12.7 mm thickness should be at least of NIJ level IV protective level). However, steel is still steel and weighs a bunch. Usually, militaries will use ceramics for their plates, to which is usually composed of Boron Carbide or Silicon Carbide, though there's plenty of experimental technologies to talk about later. Those two ceramic compositions weigh roughly the same and protect roughly the same. Concurrently the choice plate material for all modern militaries are ceramic plates of either Boron Carbide or Silicon carbide composition, and so, when you take all of that in totality, the weight of the vest with a plate will be roughly the same, only differing with the size of the wearer.
The lightest armoring material capable of stopping assault rifle rounds (7.62 x 39, 5.56 x 45, 5.45 x 39, 5.8 x 42, etc) is Polyethylene, and a NIJ level III Polyethylene plate will weigh about 1.6 kg (a steel plate capable of protecting against the same projectiles and of the same dimensions will weigh 3.6 kg while a ceramic SAPI plate which is NIJ level III as well weighs about 2.4 kg if the plate is of the same dimensions).
There's no doubt in my mind, though, that Chinese ballistic plates are probably composed of Boron Carbide, to which is the same armoring material as the SAPI the U.S. use(d). Hence why the vests weigh roughly the same.
If you want a light vest, it will have to be very bare bones minimum. Usually, the U.S. military uses such vests for their Special Forces troops, something about allowing them to move faster. The vests they use aren't really vests at all, they're simply what's called plate carriers, i.e. their only function is to accomodate a ballistic plate (usually a stand alone Ceramic plate, which weighs roughly 3.6 kg and can protect against 7.62x54 ammunition) and hold magazines. There's no kevlar in a plate carrier, and so, saves weight there. Though, without the kevlar, the user risks exposing their body to shrapnel, to which a regular soldier wearing a kevlar vest with ceramic plates would be relatively protected from.
Some comparison via "BulletProofMe.com".
A Covert style plate carrier with a 10"x12" Level IV stand-alone ceramic plate weighs 3.9 kg (3.6 kg plate, 0.3 kg plate carrier).
A Level IIIA Aramid (another strong fabric, akin to kevlar) vest with a 10"x12" Level IV ceramic plate weighs 5 kg (2.7 kg plate, 2.3 kg vest).
Obviously, if you just want to minimize weight, a plate carrier + polyethylene plate combo would be the best, as it would weigh 1.9 kg (1.6 kg plate, 0.3 kg plate carrier).
Oops, I forgot to mention that usually a soldier will have 2 plates :/
On knights though, despite it being only slightly related to the topic, that's actually not entirely true. Knights are warriors by birth, and they have worn their armor since, well, they could fit in it. They are well accustomed to wearing armor and so mobility, for them at least, becomes less of an issue than for someone who had just tried it for a day. The armor the knights wore also grew in not only size but also technology to protect the wearer from danger. Before the end of the armored knight, the English developed a triple layered armor system to which, in modern tests at least, was able to protect the wearer from a flintlock pistol and a crossbow of the same period. Obviously, arms technology soon outpaced armor technology, and only until kevlar was developed was armoring troops brought about again. But the point stands that so long as the armoring technology can keep pace with the arms technology, the armored usually wins (not always though, plenty of examples of that).