When most of the early guns were made, they were made for game shooting, or less frequently "live pigeon" shooting. The 'standard' English game set up was a gun with 2 1/2" chambers proofed for 1 1/8 oz of shot. The typical cartridge was the standard Eley Grand Prix, which was 1 1/16 oz, and would have been fairly moderate velocity. Guns would have had barrels between 25" and 30" and weighed between 6 and 7 lbs. 1 oz loads were used to get a little more velocity (Eley Impax)
For heavier use, there was the "live pigeon" gun. This had 2 3/4 chambers and was proofed for 1 1/4 oz shot. These typically weighed between 7 and 8 lbs and usually had 30" barrels. I believe the usual actual load was 1 1/8 oz.
The overall 'build' of the gun, action size, stock strength and safety margin etc was tailored to these limits.
Where heavier loads were required, a 10 bore, 8 bore or even 4 bore was used. It was generally thought that heavy loads where the shot was 'deeper' than the diameter of the cartridge gave poorer patterns.
Of course these ideas are now considered by many very dated, but they came at a time when our Victorian ancestors used to shoot a lot - and were great experimenters. I suspect there is a lot of truth in those old ideas.
