בַּהֶרֶת כַּגְּרִיס וּבָהּ מִחְיָה פְחוּתָה מִכָּעֲדָשָׁה ... רָבְתָה הַמִּחְיָה, טְמֵאָה. נִתְמַעֲטָה, רַבִּי מֵאִיר מְטַמֵּא, וַחֲכָמִים מְטַהֲרִים, שֶׁאֵין הַנֶּגַע פּוֹשֶׂה לְתוֹכָהּ:
A bright spot the size of a split bean and in it there was quick flesh less than the size of a lentil ... If the quick flesh grew larger it is unclean, but if it grew smaller, Rabbi Meir rules that it is unclean; But the sages rule that it is clean, since a nega cannot spread within itself.
It appears from this Mishnah that R. Meir (as opposed to the Sages) believes that a nega' can expand inwards and thereby become impure.
Now, consider the very similar case in 6:2:
בַּהֶרֶת כַּגְּרִיס וּבָהּ מִחְיָה כָעֲדָשָׁה ... רָבְתָה הַמִּחְיָה, טְמֵאָה. נִתְמַעֲטָה, טְהוֹרָה:
If a bright spot was of the size of a split bean and in it there was quick flesh of the size of a lentil ... If the quick flesh grew larger it is unclean, and if it grew smaller it is clean.
Since R. Meir is not mentioned in this mishnah, the implication is that he agrees with the ruling in the final clause, i.e. that where the michyah is the size of a lentil and shrinks, the nega' is pure.
Why is this so? Would R. Meir not rule that in such a case it should be impure because the nega' is expanding inwards?