Chemical spraying on fruit trees are primarily composed of herbicides to kill fungal and insect infestations. However, some of the herbicides have the additional effect of increasing growth rate in fruits and trees. One of the most common is 2,4-Dichlorophenoxyacetic acid, an herbicide that kills selectively and so is very useful for weeds and insects. Interestingly, a side-effect of 2,4-D is increased growth rate in fruits; a 1950 study by Wm. S. Stewart and H. Z. Hield shows larger fruits from the trees sprayed with 2,4-D:
On the basis of results obtained during the 1949 Valencia season from spray applications of 2,4-D in 1948the percentage of large-sized fruit was increased. From the nonsprayed trees 33.9% of the packed boxes of fruit was size 220-2%-inch diameter-and larger, whereas on those sprayed with 2,4-D, 46.2% was in this category. This is a 36% increase owing to the 2,4-D.
("2,4-D and Citrus Fruit Size" -- available as .pdf on from californiaagriculture.ucanr.org via Google)
As a synthetic plant hormone, part of the function of 2,4-D is to create uncontrolled growth in its targeted plants, causing them to die quickly. It is speculated that the 2,4-D interferes with the physiological age of the fruit, allowing them to continue growing instead of ripening and falling off the tree. This creates larger fruit faster, but may have unintended effects in the human body if ingested in large quantities.