Development is more often referred to as “functional development”, but the word has different meanings in different areas of science. Sometimes, the whole of pregnancy is called “development”, from fertilization to birth.
For example, although the brain does grow in size, especially in the first year , its functional development continues, as kids learn cognitive, language and communication skills.
Development also refers to maturation. For example the immune system “matures” as it gets better at recognizing and dealing with different types of viruses, so kids get infected less often with age.