Teacup pigs are really just potbelly pigs. Adult potbellies are about the size of a large dog and weigh ~45-100 kg / 100-300 lb.
People who wish to make money by selling potbelly piglets as “teacup” or “micro” pigs breed very young potbellies, which results in small offspring, and then keep the babies very underfed, which stunts their growth.
Buyers of “teacup” piglets are informed to only give them a quarter cup of food per day. This is starvation. Starving a baby keeps them small.
It also leaves them malnourished and weak, and susceptible to a whole range of health problems.
Teacup piglets are a scam which has lead to countless pigs being dumped at shelters.
Though they are intelligent and wonderful animals, pigs are demanding and costly to keep.
They require access to a large area of land outside, which must have a mud wallow and places to root and forage. They need the companionship of other pigs, and can be territorial, moody, and difficult to housetrain.
Breeders tell people that “teacup” piglets will stay very small and be happy in a house with no other pigs, and behave just like a dog.
When they grow to full-size, are miserable because they are lonely and starved, and behave like a pig, people abandon them at shelters.
tl;dr: they are not a breed, their size is due to starvation, they are not healthy.