A study of the views of all the Rishonim on the kezayis, as described in my monograph on the topic, shows very clearly that the Rishonim of Sefard held the kezayis to be the size of a regular olive, and the Rishonim of Ashkenaz only said differently because, as some of them explicitly admitted, they had never actually seen an olive. Not to mention the fact that all botanical and archeological evidence shows that olives were always the same size. Given all that, why are most Poskim still claiming that a kezayis is substantially larger?
I can understand Poskim who want to follow internal protocol rather than relying on science - but the views of the Rishonim themselves, such as Rashba, Ritva, Ravyah and the view in Piskei Rabboseinu SheBeAshkenaz, show that a kezayis is (shock!) the size of a regular olive. And it's not as though the large shiurim for kezayis have been canonized in either Chazal or Shulchan Aruch. Nor does it appear to be a matter of formal minhag; Poskim weigh up the views of the Rishonim and Acharonim, and draw their own conclusions.
So what is going on? Some of these Poskim are undoubtedly simply unaware of the all the Rishonim's views, and other relevant information. But I would think that at least some have been made aware of them, either via my monograph or similar information published elsewhere. So why is there no word of Poskim ruling in that way? Is it something akin to Maharam Schick's refusal to give up on metzitzah be'peh, against Chasam Sofer's policy, due to his seeing it as coming from people with a dangerous agenda? Is it caution, or fear, about issuing a ruling that appears radical and/or not frum enough?
If anyone can show my monograph to a Posek and ask for a comprehensive explanation as to why he would rule that a kezayis is larger than an olive, I would very much like to hear what it is!