There's more of us. Specifically, there's more of us contributing to human advancement than ever before. "Luxuries" like literacy are a pretty new advancement.
Population is doubled every 2 generations during the 20th century. If nothing else changed, the probability of human advancement should have been at least 2x.
There are other factors like better life expectation that speed up advancement a lot (more probabilty of having advancement per generation).
But I think the literacy factor is the key. Sadly I have no data to prove it, but I bet this was the cause of all this.
In only 116 years we've added another 6 billion to that total. :(
...and people say we're not-overpopulating the planet...
Look. Raw numbers mean nothing. What is the sustainable population of this planet. 1 billion? 10 billion? 100 billion?
I have no idea. As tech improves, our impact per person could be lower. So perhaps the limit today is 5 billion... But in 20 years it could be 20 billion.
Industrial growth, more room for more population, more manpower to evolve, loop until earth cannot sustain certain limit of population unless there's a breakthrough to feed 10s of billions.