Chief Willy
The square cube law indicates that weight increases with a higher exponential than the capacity/lift. Larger aircraft are thus structurally less efficient (payload over empty weight) than smaller ones, which negates any increase in aerodynamic efficiency (lift to drag ratio).
On a similar note, the A321neo is structurally extremely efficient, so has high payload to empty weight ratio. The aerodynamic efficiency is however a bit lower than for widebody aircraft.
Max payload / Operational empty weight (tonne/tonne)
A380: 84.0/277.0 = 30.1%
B747-8: 76.1/220.1 = 34.5%
B787-9: 52.6/128.9 = 40.8 %
A321neo: 25.5/50.1 = 50.9 %