The size of the egg referred to (this is the minimum volume of food that can convey tumah)

is neither large nor small [egg],

but rather is medium;

Rabbi Yehuda says

one brings the largest of the large

and the smallest of the small

and place each into water,

and divide in half the water (in other words, measure the total water displaced by both eggs together, and then divide in half, for the mean volume);

Rabbi Yosi says,

"and who is going to tell me

which is large

and which is small?"

Rather, it all depends on the thought of the viewer (in other words, whatever one, in their own estimation, believes to be the volume of a medium-sized egg).

פרק יז משנה ז

The size of the dried fig referred to (this is the minimum measurement for certain aspects of carrying on shabbos)

is neither large nor small,

but rather is medium;

Rabbi Yehuda says

the large one of Eretz Yisroel

is the medium of the provinces.