What is the difference to play 80Hz SINE signal with 80 dBZ SPL between Large and Small speaker？

[Chrispy]

Great question.

Perhaps the purpose is a little naive, a little silly~~~

Try to use some technical language or logic to explain the subjective feeling and common sense.

What is the “fill”？
Why the small speakers can't fill the big room?

Because they're too small to generate the necessary spl generally. Very hard to deal with comparing subjective feelings or what you consider common sense. Bigger speakers are just generally better IMO, I'd rather have bigger speakers in smaller rooms than the other way around.