Why aren't larger airliners such as the A380 more efficient than smaller ones? A recent Washington Post article, titled "Five myths about air travel" noted:


  The difficulty is that economies of scale don’t always work for airlines, because planes generally don’t increase in per-passenger efficiency as they grow larger. In fact, many of the most efficient planes of today are the smallest ones. At a transatlantic distance, a 525-seat Airbus A380 has an efficiency of 74 miles per gallon (mpg) per passenger, while the brand-new 168-seat Boeing 737 MAX 8 reaches 110 mpg per passenger.


This seems counter-intuitive. All other things being equal (and transatlantic distances would equalize many of them) -- shouldn't the larger plane should be more efficient? 

This whole article notes the "myths about air travel" and I have to admit that I'd always believed that larger planes were more efficient per passenger mile. 

Perhaps there is something unique to the the A380 and 737-Max 8 that makes the comparison incorrect? Is it related to aircraft utilization rate or percentage of seats filled -- although these wouldn't be a factor in the mpg per passenger metric?

Note: I am not interested in crew or ground staffing efficiencies realized by a mega plane. Just fuel.
The comparison is seriously flawed.
While over the entire duration of the flight the larger aircraft is more efficient per passenger/mile, this is not the case for the flight phases that cause the bulk of the fuel consumption for smaller airliners but only a fraction for larger airliners: takeoff/climb and descent/landing.

A 747 or A380 flying a one hour leg, which is typical for many smaller airliners, would use more fuel per passenger over that leg than does say a 737 or A320, despite being more fuel efficient at cruise altitude (if it can even reach its optimal cruise altitude over that distance).

It is also far less flexible, and requires a higher passenger density than does the smaller airliner, a passenger density that's simply not there on most routes unless you greatly reduce the frequency of flights (in which case you'd best hope all your competitors do the same as passengers are less likely to be happy to wait for your one flight a day at noon when they can get to their destination at 8AM taking another airline's flight).