The largest cat breed is the Maine Coon.... or is it the Ragdoll?
Mature Maine Coon males can weigh around 18lbs. and male Ragdolls anything from 15-20 lbs. They are both domestic cat breeds, the Maine Coon breed being established for over 100 years and the Ragdoll being developed in the 1960's.
The smallest cat breed is the Singapura not usually weighing over 6lbs. However, it is not a miniature cat. Indeed teacup and miniature cats are not recognised cat breeds. A teacup cat is just a miniature cat of a specific breed whereas a dwarf cat has short legs.