On another forum there is a thread where it's said that the Rigby 350 No2 cartridge pushed a 225 grain bullet at the same velocity as the 350 Rigby magnum, ie 2625fps.

I am having a little bit of trouble accepting that the information is totally accurate and I'll tell you why.
The 350 No 2 case is the same size and capacity as the 400- 350, correct? (both rimmed cases)
The 400-350 shot a 310 gn bullet and the 350 No 2 shot a 225 gn bullet.

However, the 350 Rigby Magnum, a rimless cartridge, has a LARGER case and has a GREATER capacity than the rimmed 400-350 and 350 No 2, correct?

The 350 Rigby Magnum with its LARGER case capacity produced 17.5 tons pressure to achieve the 2625 fps, in a bolt action rifle.

18.5 tons seems to be about as high as any double rifle cartridge was loaded in those days.

My question is, how could the smaller capacity 350 No2 cartridge be expected to produce the same velocity as the larger capacity 350 magnum cartridge and still keep the pressures within double rifle limits.
I mean Rigby chose to use a larger case for the 350 magnum in a bolt action rifle.
Why would they use a smaller case with the resulting higher pressure in the double?