Why a one-inch increase mattered for battleship guns

Why did Britain move from 15-inch to 16-inch guns instead of making a larger jump?

The 15-inch gun was proven, and increasing the bore to 16 inches still produced a substantial gain in firepower.

Moving from 15 to 16 inches sounds modest, but the answer begins with the proven performance of the existing 15-inch gun. Increasing the bore by one inch still delivered substantially more firepower. The change was therefore an incremental development with a useful return rather than an arbitrary new label. The brief exchange remains qualitative and does not provide shell-weight or penetration figures.