There are two ways by which the organisms increase in size. Firstly because of increase in the size of the cells and secondly because of the increase in the number of cells. For increasing the number of the cells, the cells have to multiply and copy themselves. Cells copy themselves through the process of cell division which is known as mitosis. During mitosis cells replicates and form daughter cells. The number of chromosomes remain fixed in all of the cells because chromosome number is constant in every organism. Therefore, to increase the size of the organisms cells copy and increase in number. Moreover, if cells keep on increasing their number abnormally then it can be because of some abnormality.