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As said above, there are several factors at play.
The first thing to look at is the amount of energy going into heating the block. Modern engines tend to do a better job of converting the engine in the fuel into useful work rather than heat in the block and exhaust.
What determines the amount of energy taken out by the cooling water is the temperature increase in the water and the flow rate. If you have a better heat exchanger, it will increase the temp more requires less flow. In addition, the design of the cooling system could be getting much more efficient flow requiring a smaller impeller.
There is also the possibility that the previous impeller was much larger than necessary. On engines with thermostats, you can run a very large cooling circuit which is bypassed by the thermostat most of the time.
As long as it isn't running hot, I wouldn't worry about it.
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