In the ammo world, “light” and “heavy” refer to the cartridge used, not the weight of the gun firing it.
7.62x54 is a full-sized .30 caliber cartridge, roughly half-way between the .308 and the .30-’06 in terms of muzzle energy. For a machine gun, that is a “light” cartridge. OTOH, a .50 BMG-class cartridge would be considered “heavy.”
That all said, I hope that the gun itself is extremely heavy, prone to jamming, and not durable.
“In the ammo world, “light” and “heavy” refer to the cartridge used, not the weight of the gun firing it.
7.62x54 is a full-sized .30 caliber cartridge, roughly half-way between the .308 and the .30-’06 in terms of muzzle energy. ...”
7.62x54Rmm patron vintovka obrazets 1891g is not between 7.62mm NATO and MIL STD 30-06 (formal nomenclature cal 30M2): the performance of the two US military rounds is identical, so there is no “between”. [civilian loadings of 308 and 30-06 are often higher velocity and are manufactured to slightly different dimensional tolerances than military cartridges. Do no fire civilian rounds in rifles marked 7.62 NATO, nor in US M1 Garands: serious safety risks and durability problems]
Late vintage (WWII onwards) 7.62x54R is pretty energetic, with higher muzzle velocity than US rounds and energy equivalent to German Gewehr S-patronen 7.92x57 [known in US as 8mm Mauser, but mildly loaded].
7.62x54R is now the oldest military cartridge still in military use.
The term “light machine gun” does indeed refer to gun weight. It was coined before the First World War, before anyone thought to design a mid-power “light” cartridge for military use. The US War Dept adopted the Benet-Mercie Machine Rifle in 1909.
Light is a relative term. That chunk of ordnance widely called the “Lewis Gun” (designed by Samuel Maclean, stolen by Isaac N Lewis, banned from US use by William Crozier, used by British) was widely regarded as the best light gun of WWI. It weighed more than the North Korean gun in the cited article.
Belt-fed guns firing a “full-power” cartridge were “heavy” during WWI, “medium” during WWII when heavier cartridges came in (as Ancesthntr noted - 50 cal Browning and 12.7x108 Soviet were the most common), and are now looked on as “general purpose” machine guns.
The GPMG typically weighs 21 to 27 pounds.