For the pan offering and the deep-pan offering he wipes off the excess with the thumb above and with the little finger below, and this too is the hardest service in the Temple. The handful: its minimum is indispensable to its bulk. What is the reason? The verse says "the fullness of his handful" two times.
When a Small Lack Disqualifies the Whole Handful
Yalkut Shimoni on Torah 449:4
בְּמַחֲבַת וּמַרְחֶשֶׁת מוֹחֵק בְּגֻדָּל מִלְמַעְלָה וּבְאֶצְבַּע קְטַנָּה מִלְּמַטָּה, וְאַף זוֹ עֲבוֹדָה קָשָׁה שֶׁבַּמִּקְדָּשׁ. הַקֹּמֶץ, מִעוּטוֹ מְעַכֵּב אֶת רֻבּוֹ, מַאי טַעְמָא אָמַר קְרָא מְלֹא קֻמְצוֹ תְּרֵי זִמְנֵי.