In this Halacha (Orach Chaim 486:2), the Shulchan Aruch HaRav clearly indicates that he considers a K'zayit a measurement of volume, not weight.
There, he says that the empty space between the Maror leaves cannot be counted as part of the K'zayit. On the other hand, if the item has no empty spaces, it doesn't matter how airy (and therefore lighter in weight) it is.
I was recently told in the name of R' Shusterman (Chabad Posek/Rav and Shliach to Beverly Hills) the following:
In the Luach Colel Chabad, R' Avraham Chaim Na'eh writes that a K'zayit Matzah is the weight of one ounce. The Lubavitcher Rebbe told his Chassidim many times to follow what it says in the Luach Colel Chabad, and proofread it. Since the Rebbe was aware that the Luach said we consider the weight of the Matzah, that is the way Chabad Chassidim should measure a K'Zayit of Matzah.
(I did not hear if this was to apply to a K'Zayit year-round as well, although it would stand to reason that it would).