The reference is often understood as referring to a “piece that consists of new material.” But it is more likely that the reference is to new material from which a person cuts a piece that will serve to mend a tear in an old garment. But for such work you don’t cut pieces out of new material. That’s not what new material is used for! If you do cut it from new material, then the piece sewn onto the old material will shrink (τὸ πλήρωμα αὐτοῦ) and it will make a tear in the garment as large as the piece sewn onto it. In other words, it will be larger than the tear over which the new piece was sewn. In short, new material is not meant to be used to repair old garments!1
21 No one sews a piece of unshrunk cloth on an old garment. If he does, the patch tears away from it, the new from the old, and a worse tear is made.