Shulchan Arukh, Orach Chayim 486/שולחן ערוך·אורח חיים תפ״ו

1The Measurement of a Kezayit (Olive-Sized Portion):Some say that a kezayit is equivalent to half an egg. Gloss: One should eat it all at once and not little by little. Regarding vegetables, one must compress the air spaces between the leaves and measure the kezayit based on the actual vegetable itself, excluding the air between them.