Negaim, Chapter 6, Mishnah 5. This chapter deals with the measurements of the nega and of the michyah, the patch of healthy flesh surrounded by the nega. A baheres the size of a gris is enough to place a person in hesger, and a michyah the size of an adashah (a lentil) inside it is a siman of tumah that makes him muchlat, provided the michyah is mevutzeres, properly surrounded within the baheres. Our mishnah begins with a case where both measures are generous, and asks what happens when they change.
"Baheres yeseirah mikagris uvah michyah yeseirah mikadashah", a baheres larger than a gris, with a michyah inside it larger than an adashah. Because the baheres is larger than the minimum shiur, there is ample room of baheres surrounding the michyah, so the michyah can indeed be mevutzeres, sitting centered within the nega with the required measure of baheres on either side of it.
"Ravu oh shenis'ma'atu, temei'im", if they grew, so that the michyah remains properly surrounded, or if they shrank back to the basic shiurim of a gris and an adashah, in either case he is tamei.
"Uvilvad shelo yis'ma'atu mikashiur", provided only that neither of them falls below its shiur. If the michyah becomes less than an adashah, he is no longer muchlat, though he is not simply sent home either: there is still a baheres the size of a gris, and a baheres of a gris on its own requires hesger.
That is the whole point of the mishnah's closing clause. Growth in either direction does not spoil the tumah, and shrinking does not spoil it either, so long as nothing drops beneath its measure: the baheres may not become less than a gris, which is the shiur for hesger, and the michyah may not become less than an adashah, which is the shiur that makes the metzora muchlat.