Smaller case capacity = higher pressure, with equivalent powder charges and primers and bullet.
The larger the cartridge capacity, the lesser the difference in pressures due to different brands of brass. I've weighed more than 4000 .223 Rem cases one winter from various manufacturers and cataloged their average weights. The PPU cases that weighed more than 10 grains heavier than some of the lesser weight brands of casings, gave significantly higher pressures and MVs. An additional 150 - 200 fps MV.
A 338 RUM case that weighed 10 grains heavier than another brand 338 RUM case would certainly not cause as large of an increase in pressure and MV. Because the decreased case volume in the 338 RUM case would constitute a smaller percentage volume decrease (and therefore smaller pressure increase) than in the much smaller .223 Rem cartridge case.