Because the outer perimeter must be a square. 83 and 87 could shrink the outer perimeter in one dimension, but not in both at the same time.

It is possible they can; it’s not yet proven that the listed packings for 83 and 87 are optimal.

Which of the blocks do you think you could move to shrink the solution? Or are you thinking of a completely different arrangement?

They got updated to be smaller this year, so maybe there's still more gains to be had?