In the early 20th century scientists bred together several lines of existing pigs in selective breeding programmes, mainly for laboratory purposes. Over the years many breeds were crossed which resulted in pigs of smaller sizes than their predecessors. A lot of these small pigs are, therefore, crossbreeds, with no identifiable breed parentage. They are often referred to as "micro pigs", "mini pigs", "tea cup pigs" or "nano pigs".
However, there are still recognized pig breeds which are smaller in size than the British pig varieties(Gloucestershire Old Spot and Tamworth among others). Vietnamese Pot Bellied Pigs, Kune Kune, Juliana, Swedish and Meishan Pigs are just some of the smaller breeds still in existence today. Crossbreed miniature pigs can have any of these breeds in their lineage.
Miniature pigs do not stay "tea cup" size, though. The well recognized American Mini Pig Association considers any pig between 50-150 pounds (23 - 68 kg) in weight at mature age of 5 years as a mini pig, so they are not "tiny" when adults. There are also classifications of miniature pigs according to their height which can vary from 14 - 20 inches ( 36 -50 cm). For comparison, some of the British Pig breeds can reach up to 243 pounds (110 kg) by the age of 10 months. Adults pigs can weigh anything from 300 - 700 pounds (136 - 318 kg) or more.