What made Concorde so heavy?
The Concorde mainly transported fuel across the Atlantic so that enough was available to land safely. Passengers were just an extra on top. Also, being first-class only, it held fewer passengers than what low budget airlines have taken to cramming into their planes lately.
To look at the structure relative to passengers is a bit misleading. Let's look at the fuel load and the mass fraction of empty weight relative to MTOW instead:





747-100
Concorde




MTOW
333.4 tons
186.88 tons


Empty mass
162.5 tons
78.9 tons


Fuel
183,380 liters
119,800 liters


Fuel to empty mass
1.1285 L/kg
1.5146 L/kg


Mass fraction of empty to MTOW
48.74%
42.22%




If you compare passengers plus fuel to the empty mass of each airplane, the Concorde actually comes out well ahead of the 747 and looks like a prize of lightweight design. Having structurally efficient delta wings helped and should explain most of the difference.