IBMs Many Eyes service recently published this visualisation of the world mutual fund market in 2010.
You can click on the image itself to see an enlargement, or here to see the original interactive visualisation.
The visualisation does a great job of showing just how much larger the US mutual fund market is than that of the UK and indeed even that of Europe combined.
I've argued before that size confers a distinct competitive advantage (see Innovation in Personal Financial Management: Culture or just numbers). The larger size of the US mutual fund market confers greater economies of scale, keeping prices down, and also provides a much greater prize to tempt innovators.
In order to remain competitive, Europe must provide a much more attractive regulatory environment (which comes with its own risks), or must normalise its national economic, monetary and regulatory systems in order to create a uniform environment in which its members can enjoy a combined scale similar to that enjoyed by their American competitors.