Grissom's destruction was more likely from hitting something vulnerable in the engine area (like her antimatter fuel.) But if you look at TWOK, Reliant's torpedo launcher made a nice explosion when it was hit by a low powered torpedo. The big difference, IMHO, is that a photon torpedo that isn't "powered up" will do damage but just not on the same order as one that is at "full power".
Also from "In Theory" we get a line that it would be a bad thing if a photon torpedo got partially unphased.
RIKER: It's a good thing one of these pockets didn't pass through a photon torpedo casing or the matter-antimatter containment pods.