The HK is heavy because the barrel nut is massive and the barrel profile is thick - remember the M4 barrel burst test?? It is no magic - if it needs to take more FA fire, the barrel mass needs to go up. It is not HK magic, it is physics.
Also need to consider how the piston is attached - in a SIG, the piston runs inside a gas tube , which connects the gas block to the receiver. With the HK and G36, the piston rod itself does that. Once the barrel start going soft, the piston rod itself becomes the holding support of the barrel assembly. to make sure that won't happen too early, the easiest way is to beef up the barrel.
It is a bit problematic, once the barrel start yielding, the piston may not go back into the same position as the gas block and the barrel start falling off. With the sig, the gas tube will at least keep the piston and the gas block aligned until the gas tube itself is yielding as well.