It's important to note that the torpedoes Japanese generally used we powered by compressed pure oxygen, making them faster & giving them more range.
But it also turned them into even bigger explosion hazard than normal torpedoes when the ship caring them is hit. As a result many Japanese ships are documented going down after what would normally be minor hits due to their oxygen torpedoes exploding and causing massive damage.
I wonder what the breakdown is on overall effectiveness, sometimes you can get some very perverse results. Kamikaze attacks for instance resulted in less pilots killed per damage dealt than traditional attacks even though attacks were 100% fatal, here's a good video breakdown: https://www.youtube.com/watch?v=QqwDvxLVZII
More energy stored in the same volume air tank. With normal air used by conventional torpedoes, you get just 20% oxygen and 80% nitrogen, which is not useful for combustion.
I think I might need to clarify - we are talking about a torpedo that is driven by combustion engine. It carries some sort of fuel and because it is under water, it needs to provide it's own oxidizer, which is where the high pressure pure oxygen comes in.
Also, unlike when using normal air, it should be possible for the combustion engine to be more powerful while being smaller, due to the more energetic reaction. Also the exhaust gasses don't contain nitrogen, which is apparently visible as a noticeable bubble trail, which might result in your target noticing the torpedo and dogging.
But it also turned them into even bigger explosion hazard than normal torpedoes when the ship caring them is hit. As a result many Japanese ships are documented going down after what would normally be minor hits due to their oxygen torpedoes exploding and causing massive damage.