Kelim, Chapter Twenty-Eight, Mishnah One
1) [A piece of cloth] three [fingerbreadths] square that was stuffed into a ball or was itself made into a ball is clean.
2) But [a piece of cloth] three [handbreadths] square that was stuffed into a ball remains unclean.
3) If he made it into a ball itself, it is clean because the sewing reduces its size.
Section one: By making this small piece of cloth either into a ball or using it as stuffing for a ball, the cloth loses its importance and it becomes clean from impurity. Note that this does not mean that the ball is necessarily clean. It just means that the piece of cloth that used to be unclean has shed its impurity.
Section two: In contrast, if one takes a larger piece of cloth and puts it into a ball it is still susceptible to impurity because it is still significant. However, if he sews the cloth into a ball itself, it still loses its impurity because it loses its size. A ball that is made of a less than three handbreadths piece of cloth is not large enough to be susceptible to impurity.