Question

how does a small fertilized egg develop into a big individual​

Answer 1

It is a unicellular embryo."9 (Emphasis added.) The fusion of the sperm (with 23 chromosomes) and the oocyte (with 23 chromosomes) at fertilization results in a live human being, a single-cell human zygote, with 46 chromosomesóthe number of chromosomes characteristic of an individual member of the human species.