I disagree with the way "efficiency" is calculated. "Efficiency" really should be speed/space, not speed/bc. BC is typically a lesser concern than space when building a ship, because of the high cost of the base hull type.
I would update the ship component pages for my definition of "efficiency", but I really don't feel like it.
BTW, that's why small and tiny ships are slow. For them, the size (not BC) cost of engines is disproportionately too high, forcing them to be slow.