Domestic cats are usually of a similar size (with some exceptions) because all of them, except the wild cat hybrids, originally come from the the Eurasian and/or African wildcats. These are both small wildcats that still look like domestic cats.
They were domesticated about 9 to 5 thousand years ago and nothing has really changed that much to change the size of the domestic cat since. There has been some development in selectively breeding for size in the larger domestic cats such as the Maine Coon. But the Singapura is a small domestic cat which is less than half the size of the Maine Coon. So it is not quite right to say that domestic cats are the same size.
The only big changes in domestic cat size have been in respect of the creation and development of wildcat hybrids. This entails crossing a domestic cat with a medium sized wild cat, the serval, in the case of the Savannah cat. When you introduce a fairly large cat, the serval, into the breeding program you will get at first generation (the offspring of the serval father and domestic cat mother) a very large domestic cat. The safari cat is another example.
The well known Magic, a first filial Savannah cat and the world's largest domestic, is the classic example.
Associated page: Largest Domestic Cat Breed