Transformer fuse sizing changes significantly depending on whether you protect both the primary and secondary windings, or just the primary.

Primary Only Fusing: If only the primary side is fused, that single fuse must be large enough to tolerate the transformer's brief startup current (inrush) while also providing some level of protection for the transformer and its connected secondary circuit. This often means the primary fuse can be sized quite high, perhaps 125% to 300% of the transformer's full load primary current.
Primary and Secondary Fusing: When fuses are present on both the primary and secondary sides, the primary fuse can typically be sized higher (e.g., up to 250% or more of primary current), primarily offering short-circuit protection for the transformer itself. The secondary fuse then handles the more precise overload protection for the secondary winding and its connected load, usually sized at 125% of the secondary full load current. This dual fusing offers more comprehensive and specific protection.

Transformer Fusing. Primary and Secondary