From an earlier post:
Thus at my MO seder, I divide one piece of shmurah matzah among the entire table, even though the size of each piece is far smaller than the shiur of the Chazon Ish, or even that of Chatham Sofer. I do it because my father did it that way, as did his father etc. That age old family practice, a mesorah if you will, trumps any code based criticism anyone might levy.
This is not “Modern Orthodox”, this is simply lack of concern for halacha, and motzi shem ra on Modern Orthodox to attribute to it such behavior.
No minhag can trump the chiyuv d’oraiso to eat a k’zayis of matzah; the minhag originally was to give everyone a k’zayis from the matzah of the one who broke the matzah, this was possible either because the matzah was larger, or the participants were fewer. If you want to keep the halacha and also that minhag, either use a very large matzoh, or distribute a piece from yours together with another piece, which will, combined, yield a k’zayis.