Last week SIFMA released its Research Quarterly: Fixed Income - Issuance and Trading, which provides data on US fixed income markets.
The statistics (split by issuance and outstanding) cover US Treasuries, mortgage-backed securities, corporate bonds, municipal bonds, agency-backed securities, asset-backed securities, money markets, repurchase agreements and the secured overnight financing rate.
A couple charts from the report are below. US fixed income markets comprise 40% ($42 trillion) of the $106 trillion of fixed income securities outstanding globally, and US Treasuries make up 31% of US fixed income issuance but 63% of trading. Total third quarter issuance of $3.2 trillion is up 37.5% from a year earlier, and year-to-date issuance is up over 10%.