Why do global index funds grow (beyond inflation) to reliably?

I'm intending to just start parking my money on a world index fund but want to understand the risk/reward I'm exposing myself. I understand the risk point: it's low risk as it's (1) market and secor agnostic and (2) the world's biggest companies who on average are more likely to grow rather than shrink. But as for reward, what's the logic to be confident they'll not only keep up with inflation but actually beat inflation by 4/5/6% over the long term (as people seem to be confident they will)? Surely the world's largest companies are, on average, the ones that have already grown massively and are now slow growing where any more growth much beyond inflation seems unlikely? (And yet historically they've well outperformed inflation)