While it is true to an extent that colonial powers exploited the weaker nations, the contemporary world economics are not that simple. Their former colonies now declare their independence and even excel in other fields like technology and agriculture. In digital innovations, Kenya and India are emerging, or at least venturing into, sustainable development aspects that reshape the economy of those countries.
The sad reality, however, is that even today, the legacy of colonialism contributes to economic inequalities. Exploitation of resources and people rendered many developing countries to lag behind in the process of development, and unfair trade policies are still affecting developing countries. But with global cooperation and reforms, a focus on equitable development and correction of structural imbalances, these inequalities are slowly being reduced.
Smaller nations trying to be bigger is of course what is inevitable to happen, but we have seen these other nations grow to a point where they are so powerful, they can make sure they are never down, under those unders. I mean Nigeria is growing for example, and in 50 years they will have a strong enough economy that they will maybe crack into g8, or at least be in g20 for sure. But does that mean UK will be poorer that Nigeria? Never.
So when you take what you want to take, and then leave and give people their freedom, that is not freedom, you took what they had, you can't just expect them to grow, they never had a chance to grow, you prevented them already. USA is literally the only nation that didn't need resources from others for a long time, maybe until they took oil from Iraq, but they were big by then.
And only reason for this is because they already took over a huge land, literally huge enough to give people town size farms for free as long as they farm it, and this allowed them to grow since they had what it takes to make money.