Here is a photo of a few of the common British big game or sometimes called
"Express" cartridges.

I place a 303 British at the far left for scale

left to right: 303, 500/450 Nitro, 475 No2 Nitro, 470 H&H, 505 Gibbs, 500 Eley and 577 Nitro.

I believe the 505 Gibbs was the largest bolt action cartridge ?P1010041_0031_031.jpg