Based on my personal experience trading and back testing quantitative systems, the more money you have the harder is to find alpha in the markets, for example I have one system good for up to 1 - 2M which generates me double digits yearly returns, but above that my next system for up to 10M will do only a single, up to lover double digit return (trading the stock market only, because I don't have the data to test Forex or Futures). It's all about the volume and slippage... Just can't imagine how to trade with billions without ending up owning some company by mistake :-)