Is that really true?  Probably the smallest protoconchs do occur in marine
species, but I believe the largest protoconchs are also marine.  The
protoconchs of some volutes are almost as big as full grown Helix shells.
On the other hand, some of the small, slender land snails have fairly small
protoconchs.  I would say the range of protoconch size is much greater in
marine shells than in land shells, but I don't think that land shells as a
group have larger protoconchs than marine shells.
Paul M.