No, on top of that, it's probable that a lot of these people were already living in Tallinn, yet registered elsewhere, or are still living in a nearby city, and

No, on top of that, it's probable that a lot of these people were already living in Tallinn, yet registered elsewhere, or are still living in a nearby city, and just registered with a relative/friend with an address in Tallinn. I'm not sure which would be the case more often, but depending on that it might be a positive or negative for the cities. Also, the 15% increase in public transportation would probably demand more busses/trams to be run. There should be some extra marginal costs.