Why British shells could carry larger bursting charges

Why did some British naval shells carry more explosive than comparable foreign shells?

Their casing, internal shape, explosive choice, and intended role left more capacity for the bursting charge.

Equal-calibre shells need not devote the same internal volume to explosive. Casing thickness and the intended target influence how much room remains for the bursting charge. British designers also paid close attention to explosive type and internal shaping. The difference comes from the complete ammunition design rather than calibre alone.