Giant brains in vats/tubes have been popular in anime for a while.
Dragon notes that Vegapunk's head had gotten even bigger 22 years ago: I imagine its grown since, and Vegapunk had to cut up his own brain since it simply became too big to deal with (and also so he could do more work using the other Punks).
So, this is my theory regarding the Long Head/Big Brain tribe: they can learn infinitely, but their brain/head grows as they learn.
Vegapunk's brain became too giant after he absorbed all the knowledge from Ohara, and, as his head continued to grow, he had to cut it apart to save himself from his own massive noggin.
I bet Shaka is taking the Straw Hats to show them Vegapunk's true brain: a giant piece of gray matter in a tube. It's possible his main, huge brain is running the entire island; hence the other Punks ability to communicate and work together so seamlessly.
Thoughts?
