Tosfos is bothered by the fact that this must be an absolute amount for all small vessels (as he asked on Rashi). Therefore, even when he answers that for a small vessel -- meaning one whose majority is equal to or less than the size of a pomegranate -- the amount is "Rov" of the vessel, the "Rov" cannot be referring to "Rov (most of) the amount of a pomegranate," as many small vessels barely have, or do not even have, the surface of a pomegranate! Tosfos therefore asks, what could the "Rov" be referring to -- i.e. it is the majority of what?

Tosfos answers that it is referring to a small amount, at least the size of an olive, that is missing from the vessel (and Rava's statement was made in the context of his previous statement on 95b where he discussed a hole the size of an olive). If this amount is most of the surface of the vessel, then it renders the vessel Tahor (as a vessel for solid food). (See end of Chidushei ha'Ritva, DH Ha b'Ravravi.)

All the best,

Yaakov Montrose