You miss the point - if you go back to the old DMUs they were running engines like the Leyland 600 / 680 which were 11 litres but only generating 150hp.
Now 150 hp is in the ballpark of most family cars nowadays, yet their cooling needs are far less in part because they are doing it with 1.5 - 2 litre engines.
The basic fuel / air requirements of an engine are normally greater the larger the engine is - so whilst power output may also be a factor, the reality is an 11 litre engine is going to have a greater demand for fuel and air as well as cooling demands than a 1 litre.
You are simply giving specific examples from the general thermodynamics of heat engines.
Heat engines transform thermal energy, or heat, into mechanical energy, or work. They cannot do this perfectly, so some of the input heat energy is not converted into work, but is dissipated as waste heat into the surroundings. The input heat is supplied at high temperature and the rejected heat is dissipated at a lower temperature.
In diesel and petrol engines, and in gas turbines used for surface transport or the generation of electricity, the high temperature heat energy is provided by the conversion of the chemical energy in the fuel by combustion, the useful work comes out through a rotating shaft and the lower temperature waste heat is dumped by the exhaust gases and the cooling system.
The basic theoretical understanding was developed by Sadi Carnot in 1824 and his Theorem states that the maximum efficiency of a heat engine is defined simply by the ratio of the temperature difference between the hot and the cold reservoirs and the temperature of the hot reservoir.
This theoretical maximum value is further reduced by the particular cycle that the engine under discussion uses - the Rankine cycle for steam turbines, the Otto cycle for spark ignition engines or the Diesel cycle for compression ignition engines and so on. Examples of typical engine efficiencies for current designs are 25% for petrol car engines; 35% for a high speed diesel engines; 45% for a large coal-fired electrical generating station and the largest diesel engine in the world, the Wärtsilä RT-flex96C (a two-stroke turbocharged low-speed diesel engine for large container ships which in its largest 14-cylinder version produces over 100,000 hp) peaks at just over 51%.
Taking 35% efficiency for a rail diesel as typical (and ignoring the waste heat dumped in the exhaust gases) a 150bhp diesel engine under a Modernisation plan DMU[1] will need to dump over 400hp worth of heat (nearly 300kW) through its radiators and a 1000bhp (735kW) diesel under an IET will need to dump over 2000kW through its radiators.
Note that it makes not one iota of difference how big the engine is or the number of cylinders or how quickly it runs, thermodynamics is
the defining factor in determining the quantity of heat which has to be rejected.
This is why modern designs are more sensitive to the cleanliness of the cooling system — the quantities of heat which need to be dissipated from more powerful engines are so much larger in an underfloor space which is very constrained. Simply look at the space around the engines and cooling groups of even a twin-engined Modernisation Plan DMU and compare it with that available under a modern DMU with its physically larger engines, all the emission control kit and exhaust silencers, transmissions and fuel tanks and realise that three and a half times as much heat has to be removed from less available volume.
[1] Although it should be noted that older engines were less efficient in a thermal sense than modern ones because limitations in materials and combustion processes meant that combustion temperatures were not so high.
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