As you said the main reason they gave was population; and it probably was, because servers unpopulated means less money income for them, people who stop playing because the worlds are empty, etc.
On Mar 04 2014, tibia announced their plans about the merges with this message:
Mar 04 2014 -
Game World Merges
|ith a rich history of 17 years, Tibia has seen many changes and additions, and over the years, many friendships have been forged and unforgettable memories have been made. As Tibia and its community evolved, we have also noticed that players migrated between different game worlds for various reasons which resulted in high and low populated worlds.|
Naturally, game worlds with a high population density attract even more people since they offer more possibilities to team up, connect and compete with others. Being able to play together with many other people creates an engaging and exciting play experience. It also means, though, that a massively multiplayer experience can hardly unfold on low populated worlds. This makes it difficult for players to find hunting partners or guild mates, for example, and it also reduces their chances to master group-oriented game content such as world events.
In order to provide a more enjoyable Tibia experience for players on low populated worlds, we want to address shifts of player population. Some of you have been asking us to merge low populated game worlds for quite some time already, and we have been working towards getting there. Today, we would like to give you an overview on our plans so far. While we have not yet determined when exactly game world merges will take place and which worlds will be affected, we can already provide some information on how it will be handled. Check the starting posts in our feedback thread for further details.
They even did a pool about what people thinks is un populated and these were the results:
|Below which population number of a game world do you start considering a game world to be "underpopulated"?|
|< 200||41.83 %|
|< 300||25.25 %|
|< 400||17.90 %|
|< 500||9.02 %|
|< 700||6.00 %|