Within each family, every newer generation burns less fuel per flying hour than the one before it. The figures are not adjusted for aircraft size, so they do not compare one family with another.
What this shows: The chart plots Fuel (gal/hr) on a vertical axis of Fuel consumption (gallons per hour).
Conditions and method
The bars are in the order of the aircraft families, narrow-body first and then wide-body, and run from the older to the newer generation within each family.