A map whose population was 4.8 million in version 10.27 has a population of only 3.05 million when loaded in 11.1. For each city, the 11.1 population is 63.4% of what it was in 10.27.
Curiously, its population is 3.58 million when loaded in 11.0 -- different again. (Note however that the reduction in 11.0 was not uniform, but rather varied between ~69% and ~73% of the 10.27 values.)
This savegame illustrates the difference. It can be loaded using the pakset in the block signals bug topic.
https://dl.dropboxusercontent.com/u/61716/UK2013-7.sve (https://dl.dropboxusercontent.com/u/61716/UK2013-7.sve)
Edit: I've just tested a different map and found that the population increases on 10.27 rather than decreasing. I can upload this too if it would help.
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Edit 2: some further data. All gathered in the above save.
> In 11.1 A town with only a town hall has -1 population.
> A level 1 building adds 4 population to a town (usually 6)
> A level 2 building ads 7 population to a town (usually 12)
> A level 3 building adds 12 population to a town (usually 18)
> A level 4 building adds 15 population to a town (usually 24)
And so on.
Once again, this is not uniform. In a different save, buildings give 5% more population than they usually would.
The population is intended to be interpreted from the "level" values of the building differently after 11.0 than it was in 10.27 and before, so that population numbers, town sizes and passenger generation are all in realistic proportion, so any change in these values does not by itself suggest any error.
That's good to know, thanks!
Nevertheless, it should presumably not be possible for a city with nothing but a town hall to have population of -1, as it does in the above save.
Which town hall has a population of -1?
If you build a new city and delete everything but the town hall, the city details window displays a population of -1 (and a homeless value of -9).
Perhaps the deserted town hall is used as a homeless shelter...?
I knew that Experimental had impressively advanced simluation these days, but that's just incredible! ;)