There is discussion among poskim whether we measure a kezayit (and other measurements) using contemporary olives/eggs/etc, or whether we use some larger measurement based on a perceived historical reality. See here, for example, or here (with footnotes).
There are measurements for tumah as well, such as the pomegranate-sized hole in a utensil used for storing food that makes it no longer susceptible to tumah. This measurement is listed in mishnah Keilim 17:5 right next to the kebeitsa (in 17:6) and the kezayit (in 17:8) (along with the kegerogeret and the amma).
According to those who believe that the historical reality was different, would we measure holes today in utensils according to that historical reality? Or perhaps, since a pomegranate-sized hole is one that makes the utensil unfit for its purpose, we should measure according to modern sizes, even according to those who say differently for eggs and olives?
