It is normal for birthmarks to grow as a child grows. Usually they stay the same size relative to the child, but that does mean that they get bigger.
There are many different types of birthmarks and they can behave differently. Some birthmarks can develop tumours, which will make them grow larger, but this is uncommon. A photograph of the birthmark you are asking about, along with details of who you are asking about (are they a baby, child or adult?) will assist you in...