It is possible that your tomatoes are being affected by an insect called the tomato/potato psyllid, which has a growth control hormone in its saliva. When it feeds, it stunts the growth of different plant parts, and small fruit is a classic response to these insects. To confirm this, look for the small winged insects on your plants, or the tell-tale "Lerps", which looks like sugar crystals on the leaves. For more information read: Potato and Tomato Psyllids

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