It's a weird chain reaction, it somehow makes the radius of the secondaries increase by the same multiplier. Got a small secondary that throws debris which falls 100 to 200 yards away from the target, doubled torpedo blast radius makes the same secondary bigger so the fallout splashes down 500 yards away. And the debris itself is like a bomb falling so it explodes when it hits the water or a nearby ship or boat, setting off yet another chain reaction. While experimenting with the fire damage mod I once blew up an 8 ship convoy plus two of the three escorts by going crazy with the blast radius. Chain reaction was spectacular. Fun at parties but not much use when you're trying to be realistic.
175% is as big as you really need to go, use a reasonable number of torpedoes according to the size and type of target just the 125% radius with no explosive increase will get pretty good results. The EF variable in the ZON file is the cause of many hair tearing sessions watching a carrier with 3 torpedoes in it steam off at 25 knots, if the min is 180 and the max 350 the average would be 265 hit points per hit, applied to three points with a radius of 3.5 meters with lesser damage out to 8 meters. Hiryu has 600 hit points so if you get a bad roll of the dice and all fish cause the minimum of 180 for every hit it will need 4 hits to destroy it. Larger blast radius means more flooding so more likely to sink before losing all the hit points, also slows it down or stops it with the additional flooding.