Question

why are human beings who look so different from each other in terms of size front look said to be blocked the same species

Answer 1

because of somatic variation as twins are made up of single zygote and having same phenotype but they have different finger prints due to somatic vari

Answer 2

Why are the human beings who look so different from each other in the term of size colour and looks said to be belong to same species?

There are different races of human being which different in external appearance but they all belongs to the same species.

Because all the races:

1. Share the same gene pool.
2. Are capable of interbreeding.
3. Contain the same no. of chromosome in their cell
4. Have common Molecular phylogeny involving DNA sequencing.