Babies have big heads… Why? We covered the disparity in a baby’s body proportions and their weighty heads way back in week 8 when we discovered that their heads accounted for roughly a quarter of their body weight. This week we’re looking at the head’s contents – the brain. The reason babies’ heads are so big is because they need them to contain their disproportionately large brains, which account for over 10% (according to this study by as much as 13%) of their entire weight at birth. By the time they reach 4-5 years old this will have dropped to approximately 6% and reaches the adult level of 2% around the early teens.