The answer to this question lies in tire deflection. Each tire is flattened a little under load. This creates a flat contact area.
At the same tire pressure, a wide and a narrow tire have the same contact area. A wide tire is flattened over its width whereas a narrow tire has a slimmer but longer contact area.
The flattened area can be considered as a counterweight to tire rotation. Because of the longer flattened area of the narrow tire, the wheel loses more of its roundness and produces more deformation during rotation. However, in the wide tire, the radial length of the flattened area is shorter, making the tire rounder and so it rolls better.
At 2 bar a 60mm wide tire rolls as well as a 37 mm tire at 4 bar.