SweetLou is correct, but mainly for a forced fed ICE.
Meaning the pedal (diesel's do NOT have a throttle plate) can force more than the listed displacement specification. A 6.7L can become a 8L/10L/etc...until it produces so many BTU's via more fuel injected, that the surface area of the thermal rejection system can NOT move those extra BTU's away fast enough to whatever rejection system component it has...that the metal will raise to glow, then melt
On a N/A ICE, it can ingest a bit over the displacement specification via inertia ramming...AKA scavenging effect
There are many other attributes that contribute to the BTU development of an ICE, but the above are the basic's
If you wish to delve deeper into this area, here are the 2012 GM 6.0L L29 torque/HP curves just found for this discussion. They are from the GMPowerTrain site. Mechanically, these L96's are the same. The main difference is in the state of tune. There would be differences in the thermal rejection components and sub-systems
This is for half ton pickups with L96 6.0L
This for HD pickups with L96 6.0L
This for 2500 Suburban with L96 6.0L.
This is for Express/Savanna 2500/3500 full sized vans with L96 6.0L
As a general rule of thumb, the best MPG while towing heavy is to choose an RPM just below to at peak torque. Going over that RPM will gain HP (torque x RPM / 5252), but cost MPG's and produce more BTU's
Figure what HP you desire to tow heavy at and then choose the gear that keeps your 6.0L in that range
