Yoma 39A -- Pul in Shimon's time

I do not understand what the last 10 words of Rashi on the page are talking about. Cut and paste not accuarte:

ובימי שמעון אמ מי שמגיע כפול לא היו מושטן ידיהם:

Well, Harav Yosef Berger spoke about this, and it seemed pretty complicated. I guess the place to start was Tosefos HaRosh and Tosefos Yeshanim. The question was, Can a mitzvas asei apply to less than a shiur? Also (Chasam Sofer), Can being satisfied count as a shiur for eating, even for less than a k'zayis?
But as for Rashi, isn't he saying that in the days of Shimon Hatzadik, there would have been שביעה, satiation, even with just a pul? As for why that matters and why therefore they would not draw back from taking it, I guess that depends on how you learn the other issues.