Watts is like foam on a mug of beer
it really ain’t beer..
just as Watts ain’t what’s powering your engines…it’s the Amperage
Unfortunately Watts are indeed powering your engine.
Locomotives need both voltage and amperage to roll down the track. Not enough of either and you get poor operation, or none at all. For any given transformer, from the smallest to a ZW, Voltage will fall as amperage increases, quickly in fact as you approach the maximum amperage that the transformer can produce. So if you look only at Amperage you'll end up short on performance at some point.
Looking at it another way here's the technical reason that Watts are (Power is) more important: Power = Voltage x Amperage. Unlike Amperage alone this accounts for both Voltage and Amperage limitations associated with the size of the transformer, thus is a better indicator of total performance.
If you concentrate only on Amperage you're missing half the boat.