Good question! You're right that there's never a moment where turning down a brain makes your score worse. What stops "be as big as possible" from being the whole game is the hunger: you lose a bit every second, plus about 2% of your head, so the bigger you are, the faster you shrink. A brain eaten early in the night has mostly melted away by sunrise. One eaten in the last 20 seconds keeps about two thirds of its value.

What does last from those early brains is the chain: every brain loosens it a little, for good. So the early night is really about getting the chain long enough to reach the whole graveyard while your head's still small and quick. The last half minute is where your score gets made: chain your feeds into a frenzy (back-to-back brains are worth up to 3×).

And being big costs you. You're slower, and people spook from further away. A huge head too early can leave you crawling round an empty graveyard while it shrinks.

So it's less "stay small" than "don't peak too early." Thanks for the question!