Pricing on a Citigroup issue of residential mortgage-backed securities completed on Friday indicates a gradual but steady widening of margins in the RMBS market.Citi's SAM Trust 2015-1 raised A$1.25 billion - more than twice the $500 million the bank was seeking when it launched the issue early last week.The bank will pay a margin of 98 basis points over the one-month bank bill swap rate on the $1.15 billion of A notes, which have a weighted average life of 2.7 years.Pricing on the AB and B notes was not disclosed.The loans in the portfolio have an average loan-to-valuation ratio of 64.9 per cent. Owner-occupier loans make up 73.9 per cent of the total.Earlier this month Commonwealth Bank paid a margin of 90 bps on the top tranche of its $2 billion Medallion Trust RMBS.In July ME paid a margin of 95 bps on the top tranche of its SMHL Securitisation Fund RMBS issueIn June Bendigo and Adelaide Bank paid 92 bps on the top tranche of its Torrens RMBS issue.And in May Westpac paid 80 bps on the top tranche of its $2.1 billion WST Trust issue.