Certain characteristics of land snails correlate with environmental conditions, for instance larger snails are found in more moist habitats. Pheromones of conspecifics can also influence growth rate at large population densities such that smaller snails are found when population density is high. Snail shell height is influenced by the angle of the habitat where the snail lives, this might have to do with fancy shell mechanics and balancing!
[Photo by Karli Chudeau and caption by Maggie Creamer]
Goodfriend, G. A. (1986). Variation in land-snail shell form and size and its causes: a review. Systematic Biology, 35(2), 204-223.