I talked to a Hogdon tech guy and he indicated that 2 1/2" shells have a much higher chamber pressure than do 2 3/4" shells with the same powder and charge due to the smaller explosion chamber.
I don't believe that is accurate. With the same weight/type of powder, primer, hull type and shot weight there will be no substantial difference in pressure between two shells in the same gun provided the gun is chambered for the longer of the two. There is no difference in any of the metrics except a taller wad column in the longer shell. Ask that Hodgdon guy to prove what you have told us and provide the pressure and velocity data for two identical loads in both 2.5 and 2.75 shells. Or perhaps you can provide us published data comparison in any gauge showing any significant difference in pressure between two identical loads in different length hulls. Without either you are just passing along a myth similiar to composite barrel are dangerous with nitro powders.
