Compared to the 2.3 million EU citizens in the UK, which includes people who came after Poland and nine other states joined the Union in 2004, British consular authorities estimate that 2.2 million Britons live in the other 26 EU countries, excluding Croatia, which joined in 2013.
However, more recent figures from the UK House of Commons Library Briefing Paper SN06077 show the number of EU migrants in Britain greatly outweighing those going the opposite direction. The change appears to be due to more accurate figures rather than an actual change in the migration balance.
The migration figures are available in much more detail in Migration Watch UK.
Parliament's EU Referendum page has lots more facts about other aspects of the referendum to digest.