It's pretty much implied, in the BP lore, that meson tech is much better than antimatter. Re-read the Supernova tech description, it's a meson warhead said to be "More compact than the Helios bomb, and nearly as powerful", and the Helios is antimatter.
If you count the fact that warship-mounted torpedoes need to dedicate much more space for fuel than bomber-launched bombs (that is canon too IIRC), it means the explosive charge of the Supernova is much smaller than that of the Helios, yet is as powerful.
UEF's Apocalypse antimatter torpedoes, on the other hand, do 1/5th or 1/10th of the damage of the Supernova (depending on the torpedo model). I guess you get the point.