Cleavage of the egg results in an increase in cell number, but the overall size of the organism does not increase. This period of development, in which the overall size of the embryo does not change, is short-lived. Later during development and during post-natal development there is also growth. For instance, you are not the same size now as when you were born - you have grown. Proportions of the human embryo change during gestation - i.e. the head gets smaller in relation to the rest of the body.
Differential growth can therefore have dramatic affects on the phenotype of an organism.
In addition growth could be due to an increase in the size of the cells. Growth can also be brought about by deposition of extracellular matrix such as bone or shell. Another important developmental process is apoptosis - programmed cell death. An example which explains the importance of programmed cell death during development is the hand. During development the digits form within a mesenchymal pad.