Why smaller ships do not guarantee a larger fleet

Why not buy smaller corvettes or carriers to obtain more hulls?

Because governments may authorize a fixed number of ships rather than a fixed tonnage; smaller designs can therefore mean the same hull count with less capability.

A cheaper unit does not guarantee that the same total budget will be retained for additional hulls. The Queen Elizabeth-class carriers provide an example: the navy was authorized two carriers, so building them smaller would probably have produced two less capable ships instead of a larger carrier fleet. The same constraint shapes escort totals, where class-by-class procurement decisions determine whether planned force numbers actually rise. Fleet size therefore depends on political authorization as well as unit cost.