A human is born with more neurons than they will have as adults. This is one of the main reasons that the size of the head of a child is not going to increase much as it grows. (Also, it is simply hard to make heads get bigger for various reasons, so it isn’t just humans that have large heads relative to body size when they are young.)
A person’s mass (weight) which roughly relates to volume goes up about 900% during growth. A person’s head circumference goes up about 35%. Big difference, even if you factor in the dimensional effect.
Try this: Find a small child. Preferably, your own, or if not, get permission. Show the child how you can touch your ear with the contralateral hand, by arching your arm over your head. Then, ask the child to try it. LOL.
This phenomenon, of head growth vs. body growth, comes up every time I teach about brain development, which I just did. And, it happens that right after doing that, I came across an interesting photograph. The photo is from a set of comparisons, putting an old family photo of one or more people when they were kids, to now, matching setting, clothing, props, body position, and facial expression.
When you do that, you see the head size thing really clearly in many photos. This one in particular shows it dramatically:
If you use the person’s right shoulder to help define her coronal plane, and visually project that onto the bricks, you can see that her head is close to three bricks tall in both photographs.
(I think the setting is not the same in both photos, by the way. Different bricks, different almost everything. But the person is the same, and that is what counts.)