The early 90s was mostly a perfect storm for fuel economy.

You had the computing power available to make use of CAD and develop more aerodynamic designs with less significant overhead (i.e., doing it by hand).

EFI technology had matured and carburetors were broadly defunct, allowing more efficient operation in a broader range of environments.

The US had updated its archaic lighting regulations to allow for more aerodynamic headlight shapes.

A lot of the safety technology that adds weight to modern cars either hadn’t been developed yet or hadn’t trickled down to the average vehicle.

So you had a confluence of more efficient engines, more aerodynamic vehicles, and cars that were still small and relatively lightweight.