CFTC data of clearing banks showed Citi held almost $18 billion in client margin for swaps clearing as of 30 June.
The FCM tracker provides industry-wide view of customer funds in futures and swaps.
The bank's exit could stoke further fears of consolidation in the swaps clearing market, with over half of all business concentrated with the top three dealers.
Capital pressures have seen some European clearing banks unwinding positions and retreating from the business.
Research from TABB Group estimates FCMs could record around $4.5 billion in 2016 from derivatives clearing.
Citi’s Jerome Kemp says clearing brokers must accept the new reality facing them and adopt radical changes to their models.