If you look at the graph on the link:
If you average the energies of all of the loads (excluding that low Blue line) at 2", it is just under 300 ft-lbf. Then compare that to the average at 4" which is just over 600 ft-lbf. So you get a bit more than a 100%
increase for going with a 4" compared to a 2".
When you compare the 4" to the 6" barrel it shows an energy of just under 800 ft-lbf, you get an increase of about 30%
When you compare the 6" to the 8" energy figure of about 900 ft-lbf; you have an increase of about 12.5%
Guess this is why many people "settle" for a 4" or 5" barrel as "good enough".
NRA Life Member - Orange Gunsite Member - NRA Certified Pistol Instructor
"When plunder becomes a way of life for a group of men living together in society,
they create for themselves in the course of time a legal system that authorizes it and a moral code that justifies it." Frederic Bastiat