It’s not that the Darksaber felt heaver to non-Force-sensitives, it’s that it felt heavier because Din, and Sabine, to a lesser extent, didn’t feel worthy of the blade and was subconsciously rejecting it. It’s a general property of lightsabers rather than a specific property of the Darksaber, but other blades don’t have so much meaning and cultural importance making that kind of psychological block likely to happen.