As discussed in my monograph The Evolution Of The Olive, there is absolutely no reason to think that olives in the time of Chazal or the Rishonim were any bigger than those of today. Many people are not aware of that; but even of those that become aware of it, many still use a larger shiur. Some explain that although they accept the history of the kezayis, they attribute significance to custom; Judaism is as Judaism does. I can certainly respect that policy, even if I wouldn't apply it to kezayis. However, other people claim a different reason for insisting on a larger shiur: there's no downside to it, and the benefit is that one is being choshesh lechol hadeyos, being concerned to fulfill the mitzvah according to all opinions. And especially since the Shulchan Aruch notes that some say that a kezayis is half an egg, they want to cover that view.
The idea of "being concerned to follow all opinions" is itself a fascinating topic; it very much hinges on a non-rationalist understanding of what mitzvos do. But that is a discussion for another time. The topic for today is how this plays out with maror - or rather, how it does not play out with maror.
A full discussion of the history of maror can be found in this excellent article by Ari (Arthur) Schaffer. In brief: Traditionally, wild lettuce and similar plants were used for maror. But when Jews moved to Europe, such plants were not available in the spring. As a result, they began to use horseradish instead.
However, there are a number of problems with horseradish. First, it is not in the Mishnah's list of plants/ vegetables that are approved for maror. Second, it is sharp rather than bitter. Now, the former objection is not realized by most people, due to the difficulty of translating the Mishnah's terms, while the latter does not seem to have ever been raised by Poskim. However, much more serious is that Rabbeinu Tam, based on the Mishnah, rules that only leaves and stems may be used for maror - not roots. For this reason, many authorities strongly objected to the innovation of using horseradish root. Even the Shulchan Aruch explicitly states that the root may not be used.
Nevertheless, due to the difficulty of obtaining wild lettuce and similarly leafy maror, the custom to use horseradish root became widespread. Seeking to justify this common practice, some authorities claimed that the prohibition of using roots did not apply to horseradish, whose main root is so thick that it is equivalent to a stem. But such arguments were clearly strained, and many authorities (such as Kitzur Shulchan Aruch and Chacham Tzvi) made it clear that even horseradish root was either prohibited or very much bedi'eved. (I know about the Chasam Sofer's objection to bugs in lettuce and other leafy maror. So wash it!)
Now, someone who takes the approach of striving to be yotzi lechol hadeyos should clearly be using wild lettuce or a similarly bitter leaf (I would note that today's commercially available lettuce suffers from not being bitter; but wild lettuce and endives are easy to get hold of). And there were some who did that; Rav Yosef Dov Soloveitchik, for example, had lettuce as well as horseradish. But how many people do that today? The very same people who are declare themselves to be chareidim l'dvar Hashem, striving to fulfill mitzvos according all opinions, use something for maror that is not traditional and according to many authorities is unacceptable or bedi'eved, when at no cost they could supplement it with the original maror that is acceptable lechatchilah according to all!
So why don't they eat wild lettuce, or at the very least, Romaine lettuce? It seems to me that the reason is that this idea is something that is perceived as coming from "outside." Professor Feliks, Professor Schaffer, Dr. Ari Zivotofsky - they are the "names" associated with the resurrection of this view. It doesn't make a difference that they are merely bringing to light the views of many prestigious Acharonim, which in turn are shedding light upon Chazal and the Shulchan Aruch.
Now I am not as cynical about all this as it may sound. Orthodoxy - defined by historians as a movement that began with Chasam Sofer - is fundamentally reactionary. Due to the (entirely justified) concern about wholesale abandonment of tradition in modern society, the more extreme sector of Orthodoxy has an approach of rejecting anything that is perceived as coming from "outside" - even if it is fine frum Jews quoting Rishonim and Acharonim. So it is perfectly consistent and legitimate for this consideration to take precedence over the usual policy of being yotzi lechol hadeyos.
My point therefore is not to criticize this approach. But I do think that it is important for the situation to be correctly understood.