I would say it is the other way around. The number of columns in the Foreign Key constraint matches the number of columns in the Primary Key. After all, the FK constraint refers to the PK constraint, not the other way around. A row in the child table needs to find a specific record in the parent table. In order to be able to guarantee that you can uniquely identify a record in any table, you need a PK constraint. Without a PK constraint, you may have duplicate records. If you have duplicate records, which row is the parent of in the child table?
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