It's obvious. As human brain size increased from generation to generation, your ergonomic direction would have been to pack the new mass into an oblong head perched atop the spine. It would have required stronger neck muscles and greater balance to stabilize this large head for walking and even running. It is not that a melonhead might not have been able to walk and run, certainly this design with the skull elongated is feasible up to a point. One of the limiting factors on the increase in brain mass would have been the skull being so large it was unwieldy.
Pumpkinheads would have looked ridiculous. We know because some skulls remain of this direction of development. Numerous problems with a skull shaped like a Thunderbirds puppet.
It is also apparent that as skull shape moves in this direction you increasingly become ill-suited to manual labor or vigorous movement. You're going to need to find somebody with a smaller head to do your work for you. Your mind is full of plans and you're not going to realize them by trying to do it all yourself the way a Neanderthal would. You need delegation of tasks to creatures more mobile and obedient.
Here are the other benefits of the oblong skull:
1. It was the only way the mother of a such a creature could survive childbirth. If it is a problem with the ordinary skulls of Homo Sapiens, imagine a woman trying to give birth to a child with a spherical skull.
2. Retention of ordinary human symmetry in the face, lending itself to communication and social class stratification. People would have perceived melonheads as being one of "them," except higher in social rank and superior in cognition. (Exactly as they do today, as a matter of fact, with the far less melonous) If they had been pumpkinheads it is likely that humans may have swiftly come to view them as an alien species, not a man.
3. Expanding back instead of uniformly outward, you provide for a slower evolutionary process including minor adaptations of the nerve cord in the spine, allowance for changes during growth to accommodate brain tissue movements and no need for any radical advances from one generation to the next. It is not as if the melonhead was a complete overhaul of the way the human nervous system works in order for them to survive. It permitted gradual expansion from a smaller brain mass to a larger brain mass.
4. Lastly, the wealthy can buy their way into higher social classes by paying doctors to practice skull deformation on their growing children. Throughout history, one basic principle of social organization amongst humans they insist on is the right for people with bad genes but lots of money to gain mobility into higher tiers on the class ladder through artificial means. This is why we see skull deformation practiced worldwide throughout ancient history. It is the equivalent of prep courses for the SATs. We can assume that the true melonheads were elite, buried in a remote location and considered a breed apart but this didn't stop rich kids with fake squeezed heads from mingling in the upper classes and assuming many positions of responsibility.
Again, the hard proof will come someday when the global graveyard for melonheads is discovered. This will alter all human history in an instant and confirm all the theories I have put forward here on the blog.