Why do I have very small incisor tooth?

Thank your parents. Unless you had a serious illness during the development of that tooth it is hereditary. If the esthetics are a concern see your dentist for treatment options.
Normal? The upper lateral incisors are the most misshaped teeth in the human dentition, when measured from side to side. If they are short from top to bottom you may have excess gum or what is called passive eruption. Especially if several teeth appear to short. There are plastic surgery procedures that can correct this problem. You can see a cosmetic dentist or prosthodontist for an evaluation.
Genetics. Tooth size is genetic. However in some cases patients do not have adult teeth so their baby teeth never fall out and that is the reason their teeth seem smaller, but if all your baby teeth exfoliated then you just have smaller incisors. You can fix that with cosmetic dentistry.
Microdontia. You may have a condition called microdontia. Some people are born with smaller than normal teeth. Microdontia of the lateral incisors is quite common. This condition can easily be corrected with porcelain veneer treatment.
Genetics. Blame this one on your mom or dad. It is a common congenital issue. If it bothers you, see your dentist to discuss some cosmetic options. Many different materials are available to make your teeth appear a more ideal size. Composite bonding or ceramic veneering can create the size, shape, and color that you desire.