Belgium indeed went on a bit of an exploitation spree. But it's never been notably wealthier than its neighbours. (Of course the king got some nice palaces, but that's not country-level wealth.) Instead the industrial part of Belgium was wealthy in the same era that the nearby industrial part parts of France were (and Detroit was). Now that's rust-belt poor, and the wealthy bits are tracking Holland. These patterns seem hard to explain by referring to the (very real) evils of the congo empire.
It's doubtful Congo was responsible for the wealth of Belgium.
- What is now Belgium was one of the wealthiest parts of Europe in the Middle Ages, long before any colonization.
- Belgium was the second European country to industrialize. This caused the economy to boom and Belgium had the 4th highest GDP in 1900. Congo was only a colony since 1908.
- In 1918 the whole economy was destroyed because of the war. The Germans had plundered the country. Whole factories were dismantled and shipped to Germany.
- In 1945 the economy was once again destroyed because of the Germans.
- In 1956, at the heigh of colonial exploitation, Congo was 3,3 % of Belgian GDP and 75000 direct and indirect jobs. Not nothing but hardly the base of all wealth. It was much lower in the preceding decades.
- Congo became independent in 1960. Annual GDP growth rates were much higher during the 60's than during the whole colonial period.
I would say Belgium is currently wealthy because of the Marshall plan.
Which countries are you referring to? First of all borders in Europe were not the way they are right now 150 or 200 years ago.
Second, an example for a tiny country that was exploiting an african nation well into the 20th century is Belgium [1]
[1] https://en.wikipedia.org/wiki/Belgian_colonial_empire