Nope, the tables aren't copies of each other. They are unique, they simply contain an identical property between them that the referential integrity has to work with. The remaining data can be anything you want them to be. The related properties DO need to be identical in order to apply a constraint though. Identical definition will result in identical number of bytes required for each of the two.
Number of fields is irrelevant, one table can have a single lookup field and another can have 15 fields with only a single one related.
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