If you have Fredo 8 inch Single Magnet subwoofer, you may go for either of the first 2 screenshot data which you attached with your question.

If you have Fredo 8 inch DVC with you, with double magnet, you may go for the smaller box size as power requirement on amplifier is increased.

Please note a bigger box enhances bass characteristics for the same subwoofer. Box size is kept slightly higher if amplifier power is inadequate/less.

As you move towards more professional AV receivers/Amps, box size can be reduced by few litres for the same subwoofer size.

Hope this explains.