You are right; the largest gains in global economic output where made after the industrial revolution. http://www.economist.com/blogs/dailychart/2011/06/quantifyin...
But you can't ignore British policies that decimated India's manufacturing capability before that. Like what's been written below..
It was just pure ignorance. Some of the larger impacts of colonialism:
Indian share of world economy went from 23-24% before colonialism to less than 4% under colonialism. India's share of World Trade went from 27% to 2% under colonialism.
India's healthcare worsened during colonialism because all the resources were exported to Europe for cheap and in abundance
India's literacy was worsened during colonialism and was just close to 10% during Independence.
India's revolutionaries who opposed colonialism and raised against the British were brutally killed or hanged under British rule.
Anyone who spoke against the colonialism were jailed.
British Divide and Rule policy still affects India today.