OP is talking about "share of the world's wealth," not a fixed pie. While the raw amount of wealth (the pie) in the world does grow, the important measurement is what percentage of that pie is captured by the rich. And the rich are capturing an increasing percentage of the growing pie. That's where the inequality lies.

In a world of two people, where today I have one piece of bread and you have two, then tomorrow I have two pieces of bread and you have 10, I am worse off.