"Most of the tomatoes we encounter in the supermarket are high yield strains, harvested when "mature green," which means that fruit has developed enough to support continued maturation once it's off the vine, but not so much that they'll bruise easily or rot during shipping (mainly from Florida, California, and Mexico).
Picking "mature green" is the optimal way for growers to transport massive amounts of fruit so that there's always a supply, 12 months out of the year, no matter where you live. But this approach rarely produces aromatic, flavorful tomatoes—that's why we recommend growing them yourself or shopping at your local farmers market for the best flavor and texture. The one exception to this rule? The smaller the commercially grown tomato, the better it's likely to taste. That's because smaller fruit is less likely to bruise during transit, and is therefore allowed to stay on the vine longer, plucked when it has actually started to ripen."