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My idea is that it is most directly a result of their large displacement.
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Absolutely, there is no substitute for displacement in terms of power production.
That is what makes the article Viper posted and I quoted/linked so interesting.
It uses piston speed instead of crankshaft speed to attempt to compare the engines and fuel metering of the Rolls Royce single point injection vs the German Direct Injection systems under the same conditions. The article unfortunately does not compare the float type carburetor used during the Battle of Britain. If it did, there would be an even greater difference.
The result is at the same piston speed, the direct injection has a lower specific fuel consumption at any given brake mean effective pressure. That means it produces more power for a given amount of fuel.
It also shows an interesting graph comparing the Merlin IF it had the same compression ratio. At the same compression ratio, it is equal or better than the Jumo engine. Of course that is IF it had the same compression ratio...which is pure speculation but interesting from an academic standpoint. If frogs had pockets, would they carry pistols and shoot snakes?
A simple fix that is not so simple. Simply raising the compression ratio of the Merlin is as complicated an engineering problem as those who ask "Why didn't they just increase the wing area to lower the wingloading?"