|Comparison domestic and wildcat ancestor.|
There are other small wildcats who are also suited to domestication to various extents e.g. the margay, but it just happened that the North African wildcat got their first perhaps because 10,000 years ago the Middle East was a developing and active area for farming. Farmers would have encroached onto the wildcat's territory forcing them together. There may have been conflict between human and cat but some farmers liked their presence for their rodent killing habits.
There must have been the very first domestic cat. At that time there was a single domestic cat in the world. Now there are around half a billion domestic and feral cats.
It is worth arguing that the question is incorrect formulated as what is small? There are smaller animals than the domestic cat and there are larger ones. They are small compared to us. It might be argued that the domestic cat is somewhere in the middle range of animal size. Therefore it could also be argued that they are not small. I think perhaps the reason why the question has been formulated is because the questioner is comparing the cat to the size of humans. Therefore relative to humans the domestic cat is small. That is not a good reason to ask, "Why our domestic cat so small?"