The U.S. financial
While this may be true on an absolute basis, it isn't so much relative to GDP.
In a recent report, the McKinsey Global Institute charted the sizes of country stock and bond markets and measured it relative to GDP:
To compare the size of financial systems, we calculate financial depth at the country, regional, and global levels. This is calculated by dividing the financial assets of a country or region, as defined above, by its GDP. There are large variations across countries, and in general countries with higher income levels have deeper financial systems (Exhibit A2).