Mekor mekomo tamei

Even if you hold mekor mekomo is tamei it is still just tumas maga so why does Rashi (nidda 41b d"h rabbonon savri) say it would be metamei b'kol shehu? Doesn't tumas maga always have a shiur?

Same quesion lower down on nidda 41b when R' Yochanan says that mekor sheheziya k'shtei tipay margoliyos is tamei tumas maga. Why are two drops tamei (and the gemara explains that even one drop would be tamei except that we can assume it didn't come from the mekor), isn't it less than the shiur?