Why Type 26 variants can differ greatly in displacement

How similar can the City, Hunter, and Canadian Type 26 variants be when their full-load displacements differ?

They remain similar through a shared architecture; national hull size and equipment can vary because the expensive common value lies chiefly in design and systems integration.

Common architecture does not require an identical finished ship. The underlying arrangement and integrated systems carry much of the costly design work. Steel and added hull volume are less decisive than developing how the ship's equipment communicates and functions together. This allows each navy to enlarge or alter its variant while retaining the economic benefit of a common family.