By some clever use of normalization and a few lines patch for ModRana I have been able to slim dataset for the whole of Poland with zoom level up to 15 and select cities up to 18, from 4792M to 4589M. That's the whole 203 megabytes less, with no performance impact.

I also wonder how much it would be squeezed further by using png16 instead of png256. It would, of course, require a new low-color stylesheet too... And some tweaks to renderer