Offfhand;
It's like this; When using a heavier bullet the bullet is of course longer for any given bore diameter. The .243 and the 6mm share the same bore size regardless of the nomenclature designating the cartridge. So, a cartridge with a shorter neck, seating a longer bullet, has the base of the bullet down in the boiler room taking up space that could otherwise be used for powder. Therefore, with the 6mm having a larger case anyway, and not having heavy bullets taking space from powder, it's easy to get more velocity with heavy bullets. Not at all unheard of to gain 200 fps over the 243.
When you do something like using the Speer #1217 90 grain spitzer, it's almost like finding another 50 horsepower in your car. That bullet in the 6mm can, depending on your gun and other variables, gain 300 fps over a 100/105 grain load from a .243.
900F