Jesus describes quite a number of variables here, and we could add such things as weather, insects, viruses, molds, parasites, soil quality, and seed quality.
What happens when we plant an apple seed? In due time, the seed grows into a mature tree, which produces far more than just one apple. In all likelihood, it will produce many bushels of apples for many years. The apples fall to the ground or are carried away from their point of origin—sometimes very far away in the stomach of a bird or a horse. New seeds are then deposited on the ground, and the "spreading" cycle begins anew to be repeated almost endlessly. Of course, there are some impediments to this process, but where the factors are right, both apple trees and their fruit can increase greatly.
Suppose someone plants a choice morsel of gossip into another's ear. If that sin falls on fertile "ground" (a person with all the "right" proclivities for carrying it to others without regard to consequences), who knows how much destruction can be caused! If that person tells ten others, and these in turn tell ten more, in three cycles one thousand people are involved in this sin! It is entirely possible that not even one person in that thousand would see himself as a cog in the process of spreading potential destruction!
Paul confirms this in II Timothy 2:16-17. "But shun profane and vain babblings, for they will increase to more ungodliness. And their message will spread like cancer." The New English Bible translates that last sentence as, "The infection of their teaching will spread like a gangrene." Conduct like this will bear bad fruit because human nature provides very few impediments to sin. Human nature can produce nothing else, as Paul writes in Romans 8:6, "For to be carnally minded is death." To add to the tragedy, human nature almost always drags others into its curse along the way to death.