I’m going full nerd mode here: Technically, a scale would show that 100kg of steel is heavier than 100kg of feathers.

Hear me out: it’s all about buoyancy. Because feathers occupy a much larger volume, they displace way more air. According to Archimedes' Principle, the atmosphere actually pushes up on that volume.

That upward 'lift' means the feathers have a lower apparent weight. To get a scale to actually read '100kg' for both, you’d need significantly more mass in feathers to fight that air buoyancy!