Larger shells may reflect several causes, including:

Longer growing season-likely in species in the middle of their range.
More food
Less effort put into reproduction (e.g., temperature often too low to breed in a given year; parasitism that attacks the gonads)
Greater age (for species with indeterminate growth)
Lack of stresses that make other individuals smaller
Sorting by waves and currents that favors deposition of larger shells
Fewer tourists picking up all the big shells

    Dr. David Campbell
    Old Seashells
    University of Alabama
    Biodiversity & Systematics
    Dept. Biological Sciences
    Box 870345
    Tuscaloosa, AL  35487-0345 USA
    [log in to unmask]

That is Uncle Joe, taken in the masonic regalia of a Grand Exalted Periwinkle of the Mystic Order of Whelks-P.G. Wodehouse, Romance at Droitgate Spa

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