I noticed the same thing. You could have acheived the same level of damage by launching rocks the size of those torpedos.
If i punch my wall i create a hole, unless my fist has explosive properties i have no reason to assume the entire wall would come down, and that is why we build explosive devices... to cause more damage than the actual size and mass an object can provide. Borg torpedos appear not to work on that principle.
UNLESS they were bunker buster torpedos and exploded deep underground........ yes........ thats it......Borg Bunker Buster Bombs (Say that 10 times fast)