The Gemara says that every single cup has to have it an amount of a Revies. The Rashbam explains that is derived from a Pasuk that tells us that less than a Revies is not considered 'Shteeah'. However the Gm' later says that one can fufill his obligation of drnking 4 cups as long as he has ROV Kos, but if it's less than a Revies, how can it help if he drinks Rov Kos?
Yisrael Apfel, Bergenfield,NJ
(1) The answer is that we always say "Rubo k'Chulo" -- the majority is considered to be the complete entity (see, for instance, Horayos 3b, where the Gemara states that for all Torah matters, the law is that "Rubo k'Chulo"). Therefore, if one drank the majority of a Revi'is, this is considered equivalent to drinking an entire Revi'is.
(2) This is stated by the Bach on the Tur OC 472:6 (DH u'Mah) who writes that the reason why one should drink an entire Revi'is l'Chatchilah, and only b'Di'eved does it suffice to drink the majority of a Revi'is (this in fact is also stated by Tosfos here, 108b DH Ruba), is that we apply the rule of "Rubo k'Chulo" only to be Yotzei b'Di'eved, while l'Chatchilah one must drink the entire Revi'is. One learns from this that the reason why Rov Kos suffices is because of the principle of "Rubo k'Chulo."
(3) This idea is also set forth by the Brisker Rav in Chidushei ha'Griz on the Rambam (Hilchos Chametz u'Matzah 7:7 DH veha'Nir'eh) who writes that since the basic Mitzvah is to drink the four cups, it follows that one must drink the entire cup. However, because of the rule of Rubo k'Chulo it suffices to drink the majority of each cup.
Dovid Bloom