Here, the applies to world, not to world's largest.

This is the world's largest bubble bath.

Consider the substitution of a local context instead of global:

This is our largest fork.

Of course, the is not the only article that could help here, depending on the context and intention.

In a world's largest city, there are usually districts.

 

My world's largest gelatinous cube has 4,000 hit points.

Here, the applies to world, not to world's largest.

This is the world's largest bubble bath.

Consider the substitution of a local context instead of global:

This is our largest fork.

Of course, the is not the only article that could help here, depending on the context and intention.

In a world's largest city, there are usually districts.

My world's largest gelatinous cube has 4,000 hit points.