Why is the koalas size less than 1 inch long?

Koalas are not actually less than 1 inch long; they are typically about 24 to 33 inches in length as adults. Perhaps you are referring to a specific stage in their development, as newborn koalas, called joeys, are very small at birth, measuring around 1 inch long. They spend several months developing in their mother's pouch before emerging and growing to their full size.