The Rambam (Chametz u'Matzah 6:1) formulates the mitzvah of achilas matzah as follows:
מצות עשה מן התורה לאכול מצה בליל חמשה עשר שנאמר בערב תאכלו מצות. בכל מקום ובכל זמן. ולא תלה אכילה זו בקרבן הפסח אלא זו מצוה בפני עצמה ומצותה כל הלילה. אבל בשאר הרגל אכילת מצה רשות רצה אוכל מצה רצה אוכל אורז או דוחן או קליות או פירות. אבל בליל חמשה עשר בלבד חובה ומשאכל כזית יצא ידי חובתו:
The Rambam could just as easily have said מצות עשה מן התורה לאכול **כזית** מצה בליל חמשה עשר Why the circuitous מצות עשה מן התורה לאכול מצה...משאכל כזית יצא ?
B'pashtus one would have thought that once you eat your kzayis of matzah, the mitzvah is over and if you continue stuffing your face, that's for your own pleasure alone. But it seems from the Rambam (and this is also the opinion of Maharal in Gevuros Hashem ch 48) that that's not how it works. There is a mitzvah to eat, period -- the more, the better. Every additional bite is a kiyum. If you want to do the minimum, then the Rambam at the end of the halacha tells us that you are yotzei with a k'zayis.
The gemara (Pesachim 107) relates that Rava used to drink a lot of wine on erev pesach so that he would have an appetite to eat a lot of matzah during the night. Rav Wahrman (She'eiris Yosef I:1) brings proof from this gemara to the Rambam's point. Had the mitzvah been to eat a k'zayis, there would be no point to eating more.
(Tos in Kiddushin 38 writes that you can't say aseh doche lo ta'aseh when eating a second k'zayis, but that's because dechiya depends on their being a chiyuv, not just a kiyum.)
The gemara (Sukkah 41) similarly writes that there were people who held their lulav and esrog all day, even while davening. The gemara asks how that can be when there is an issur of holding things in your hand during tefilah, and the gemara answers that taking the lulav is different because it's a mitzvah. Even though m'ikar ha'din a person is yotzei netilas lulav just by picking it up, we see from the gemara that that's just a minimum, like eating just a k'zayis of matzah, but if you do more, it also counts as a mitzvah.
The gemara (R"H 16) writes that we blow shofar during shmoneh esrei on R"H even though we already did the mitzvah before starting musaf in order to confuse the satan. Rashi comments that by blowing extra times we demonstrate our love of the mitzvah. In his sefer on chumash (shmos p50) Rav Noson Gestetner suggests that you see the same chidush from this Rashi. Had there been a defined shiur beyond which there is no kiyum, by blowing extra kolos we would just be entertaining ourselves. Rashi is telling us that the additional kolos are a mitzvah, as there is no maximum limit to "yom teru'ah."