Munchkin cats will normally stay very small even when they are full grown adults.
A Munchkin cat has short legs and will usually always resemble a small kitten because of the short legs even when they are full grown.
Once the Munchkin cat gets past a year old they will always stay the same size they are are currently because they won't grow anymore than that.
The Munchkin cat when full grown will weigh between 5 to 9 lbs and will not grow any bigger than that and they are very cute cats and will seem like you have a kitten the entire life of the cat.
Although they are full grown but resemble a kitten and as long as you love them and show them love and care for them properly they will show the love and affection back to you.