Within the same snail population and under the same conditions, some snails will grow faster than others. Some will take twice as long to mature. This may help the species survive bad weather, etc., in the wild. However, a snail farmer should obviously select and keep the largest and fastest maturing snails for breeding stock. Sell the smaller snails. By selecting only the largest, the average size of the snail may increase significantly in only a couple of generations. Most of the differences in growth are probably due to environmental factors including stocking density. However, to whatever extent these differences are genetic, you might as well breed large, fast-growing snails instead of small, slower-growing ones.
Several factors can greatly influence the growth of snails including: population density; stress [snails are sensitive to noise, light, vibration, unsanitary conditions, irregular feedings, being touched, etc.]; feed; temperature and moisture; and the breeding technology used.
H. aspersa requires at least 3% to 4% calcium in the soil (or another source of calcium) for good growth. Most snails need more calcium in the soil than H. aspersa. Low calcium intake will slow the growth rate and cause the shell to be thinner. Calcium may be set out in a feeding dish or trough so the snails can eat it at will. Food is only one calcium source. Snails may eat paint or attack walls of buildings seeking calcium, and they also will eat dirt.
A newborn's shell size depends on the egg size since the shell develops from the egg's surface membrane. As the snail grows, the shell is added onto in increments. Eventually the shell will develop a flare or reinforcing lip at its opening. This shows that the snail is now mature; there will be no further shell growth. Growth is measured by shell size, since a snail's body weight varies and fluctuates, even in 100% humidity. The growth rate varies considerably between individuals in each population group. Adult size, which is related to the growth rate, also varies, thus the fastest growers are usually the largest snails. Eggs from larger, healthier snails also tend to grow faster and thus larger.
Dryness inhibits growth and even stops activity. When it becomes too hot and dry in summer, the snail becomes inactive, seals its shell and estivates (becomes dormant) until cooler, moister weather returns. Some snails estivate in groups on tree trunks, posts, or walls. They seal themselves to the surface thus sealing up the shell opening.