The gemara quotes a mishna in challah to prove that ta'am k'ikar is so powerful that on a Torah level it converts the entire mixture (even if it is a minority) to be like it both for an issur to be chayev malkus, and by a mitzvah to qualify as eating a kezayis. The Mishna says that if one would bake matzah with for example 20% wheat flour and 80% rice flour, they could fulfill their mitzvah of matzah with it. The Shulchan Aruch (O.C. 453:2) codifies this, but it is unclear how large of a piece must be eaten. The M.B. explains that there are various opinions regarding the nature of rice flour turning into chometz when mixed with wheat flour, and therefore even if there would be only a small quantity of wheat flour (less than kezayin in achilas p'ras) one could fulfill their mitzvah of matzah, but if the wheat flour were mixed with corn flour there would need to be at least a kezayis of wheat within an achilas p'ras. However, almost no opinion (except for the Ra'avad mentioned in sha'ar hatziyun) requires one to eat multiple kezaysim of the matzah to ensure that he will be eating a kezayis of wheat, rather it is sufficient to simply eat one kezayis. Even the strictest opinion mentioned in the M.B. would hold that if there is a majority of wheat flour to corn flour, we view it as if it were entirely wheat flour so that one can fulfill the mitzvah by eating just a kezayis.
Tosafos asks why the gemara says in Pesachim 115a that one shouldn't eat maztah with marror because the marror which is only d'rabonon will be mevatel the matzah which is d'oraysa. Being that our gemara merely requires the consumption of a kezayis which has the taste of matza, and not to eat a kezayis of matzah, one should certainly be able to fulfill their mitzvah when they eat marror with the matzah? Tosafos answers that specifically by marror which due to its sharpness ruins the taste of the matzah do we consider it to be a problem, whereas the rice flour doesn't ruin the taste of the wheat flour. Secondly, Tosafos answers that the marror ruins the taste of a "kezayis matzah", meaning that if you only eat a kezayis of matzah with the marror you end up tasting less than a kezayis, but if you ate a few kezaysim with the marror you would fulfill your mitzvah.
The Minchas Chinuch (Mitzvah 6 - eating korban pesach) proves from the first answer of Tosafos that one can fulfill their mitzvah of matzah or korban pesach even if they were to eat something else together with the matzah, so long as it wasn't sharp like marror (so long as it is a different type of item, to the exclusion of eating regular meat together with korban pesach). However, the second answer of Tosafos would seem to reject this and hold that even eating an item together with the matzah that isn't sharp could prevent one from fulfilling their mitzvah of matzah - it would all depend on the quantity that they are eating. The M.C. points out that if this were true, the gemara doesn't make sense. The gemara asks on Reish Lakish who holds that one issur is mevatel another indicating that taste isn't d'oraysa to maintain the status of each issur, from the case of mixing wheat and rice flour indicating that taste is strong enough to consider the entire loaf to be matzah. Why doesn't the gemara answer that Reish Lakish is speaking when one one kezayis is eaten and the mishna is speaking when you eat multiple kezaysim? Therefore, the M.C. explains that Tosafos maintains there original position that only the sharpness of the marror can ruin the taste of the matzah, but adds a stipulation that even this is only when one kezayis of matzah is eaten - if multiple kezaysim were eaten, even marror wouldn't be mevatel the matzah. See Minchas Chinuch for a summary of these dinim.