A giant vegetable is one that has grown to an unusually large size, usually by design. Most of these maintain the proportions of the vegetable but are just larger in size.
Vegetables most usually grow into an unusual shape due to environmental conditions. Damage to one part of the vegetable can cause the growth to slow in that area while the rest grows at the normal rate. When a root vegetable is growing and the tip is damaged it can sometimes split, forming multiple roots attached at one point. If a plant is in the primordium (embryonic development) stage damage to the growing vegetable can cause more extreme mutations.
The unusual shape can be forced upon the vegetable. In Japan, farmers of the Zentsuji region found a way to grow cubic watermelons by growing the fruits in glass boxes and letting them naturally assume the shape of the receptacle. The square shape supposedly makes the melons easier to stack and store, but the square watermelons are often more than double the price of normal ones. Using similar techniques growers have also created more complex shapes of watermelon, including dice, pyramids, and faces.
Some giant vegetables have been purposely cultivated to be of an enormous size, with artificial selection being used to create large hybrids. Serious growers (those that enter competitions and aim for world records) trade seeds through clubs or over the Internet.
The BBC comedy television programme Blackadder contains several jokes relating to the character Baldrick and his obsession with odd shaped turnips. The most notable example being in the episode Beer, in which Baldrick discovers a turnip shaped exactly like a "thingie", forming several jokes throughout the episode.
Unusually-shaped vegetables feature in a Terry Pratchett novel "The Truth", where a man constantly submitting them to a newspaper for publication is a minor but frequent annoyance to the paper's editor.