But Chang had (fake or Cartwright-provided?) Starfleet torpedoes, so that he could frame the assassination on Kirk.
If Cartwight provided those torps, they probably were weaker than their regular Starfleet equivalents: Chang only initially had use for a disabling yield, and Cartwright might make sure the torps were incapable of taking bigger yields. If the Klingons manufactured them themselves, then their lack of expertise might have limited the torps to disabling yields, explaining the drawn-out final fight.
Without a crew to control the variable yield of the torpedoes on Enterprise, they're all still set to what they were last loaded with.
Hmm. I sort of doubt torpedoes can be kept loaded for very long, as what they are loaded with is antimatter. The big starships struggle to contain their antimatter stores; the tiny, expendable torps might do more poorly in that respect.
It might be that a crewless ship is limited to injecting a minimal amount of AM into the torps, and supervising crew would be required (if not for practical reasons, then at least by the regulations) for increasing the yield, and Scotty didn't think of removing the programming blocks that enforce these limiting regulations.
Or it might be that Kirk's torpedo system is inherently weak: much of the automation has been ripped out or shut down to give the trainees more to do, and for their own good the potency of the weapons has been ramped down a lot, too.
Personally, I favor the idea that TOS movie torps are as powerful as TNG ones, more or less - and as lethal to their own users if fired point blank at high yield. So Scotty chose a low yield, more or less sufficient for knocking out the BoP without harming the weakened
Enterprise, and erred ever-so-slightly on the side of caution.
We never see Khan fire one of his own torps in murderous anger: the first fight involves him wanting to wound Kirk, then he wants to warn Kirk of entering Mutara, and then when he finally would have a motivation to fire at lethal yield, he misses. So we can't readily compare the torps of the two starships, thankfully enough.
Timo Saloniemi