T-shirt, I think there are two totally different issues here:
1) the number of locations
2) the distances between those locations
The Polish area probably covers at least as many locations as the English, cities have similar sizes, and they have reasonable sparsely inhabited areas on their borders. This has not done much bad for the game - in fact, they have a very lively area.
The old English islands have far fewer locations than most other islands (except the really small ones), but with large distances between them, and the game seems to be far less exciting.
So we're not talking about removing locations - indeed, there are thousands, and still large numbers of them unexplored. But we're talking about reducing distances, one way or another, or, alternatively, chasing people away from the islands with the large distances

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I am with HF and really like this latter idea, but expect big player revolts

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We could of course reduce all the road distances on the two old islands, since they are stored separately from the actual locations, but that will hugely increase the deviation in actual distance / travel distance, and land distance / sea distance. So sailing maps wouldn't change, nor would the map, but the ratio walking/sailing will be very different from other islands. It was mentioned above that this is already somewhat the case that on those islands there is a mismatch, but of course not this dramatically.