In general, domesticated food plants have larger fruits, heads of grain, tubers, etc, because this is one of the characteristics that early hunter-gatherers chose when foraging for food and later planting it.
Domesticated tomatoes can be up to 1000 times larger than their wild relatives but how did they get so big? While tomatoes have long been bred for shape, texture, flavor and nutrient composition, but it has been difficult to study these traits in tomatoes, because many of them are the result of many genes acting together.
These genes are often located in close proximity on chromosomal regions called loci, and regions with groups of genes that influence a particular trait are called quantitative trait loci (QTLs). When a trait is influenced by one gene, it is much simpler to study, but quantitative traits, like skin and eye color in humans or fruit size in tomatoes, cannot be easily defined just by crossing different individuals.