Belgium has an unusual constitution that lets its regions have veto power over some of its decisions in the international stage and adding a member to the EU is actually a change to a major Treaty that Belgium is part of.

For most EU member countries, there is no such thing, though I believe some (Luxemburg, Malta?) are actually smaller than Walonia in terms of population.

It's my impression that it's actually a lot more about national pride for Spain than about Gibraltar's fiscal paradise status, since Gibraltar as not part of a member country can just be treated the same as any other offshore fiscal paradise, such as the Bahamas, which includes it being added to black lists. In this day and age, it's not geographical proximity that matters when it comes to fiscal paradises.

This makes sense since Britain too doesn't really gain much from having possession of Gibraltar so holding on to it is mainly a question of national pride for the UK - it would be strange if Spain's motivations were wildly different.

PS: Also it's funny how during the Leave campaign a lot of the "reason" why the EU would give Britain quasi-membership rights (without the responsabilities) after leaving the EU were a lot like this, about how those other countries or interests inside those countries would do it because they stood to gain monetarilly from it in the short term. All that turned out to be mainly wishful thinking and a serious misreading of the motivations of the leaders and people in said other countries.

Just found it funny how there are still people around thinking other countries are mainly motivated by the short term gains in sovereignty affairs, even whilst Britain itself again and again keeps doing things motivated by national pride when it comes to such affairs - one would've expected that "they're a lot like us" would somehow been figured out by now.