I think the 25% number is a bit of propaganda. It obscures the fact that primary reason Indian GDP as share of global GDP fell is that European nations industrialized and rapidly grew their populations.

Same thing is happening to Europe today. Our GDP is falling relative to world GDP and Indian and Chinese share of world GDP is growing. Does that mean that Europe is being exploited and made poorer by India and China? Of course not!

India had similar GDP per capita to China once liberated and if you look at countries not colonized there was not much difference. Many problems with colonization but it didn't have quite the negative effect on the economy as many would like to think.

For instance Britain build railroad in India and improve communications and road networks in general which improved trade and goods transport within India significantly.