According to the firm’s half-year results, the acquisition of Virgin Money has added £2.3 billion to nationwide.
Building Society completed its £2.9bn takeover. Of Virgin Money in October, making it the UK’s second largest mortgage and savings provider.
The acquisition brings together approximately 24.5 million customers, more than 25,000 employees and nearly 700 branches.
Both brands will remain on UK high streets for between four and six years, before Virgin Money is fully absorbed across the country and converts consumers.
Britain’s most senior female banker, Debbie Crosby, Nationwide’s chief executive officer, said the profits from Virgin Money would now be used to pay customers rather than paying shareholders.
He said the leverage his company gained from buying the rival for less than its £4.4bn valuation provided an opportunity to invest in service and value.
The firm’s half-year results, published on Wednesday morning, showed record growth in mortgage and retail deposits across the country and the highest ever member value at the half-year stage.
This includes £950m of refunds in rates and incentives, as well as £385m in Fairer Share payments, which look at rewards paid to eligible members.
Ms Crosby said: “Over the past 18 months, our mutual model has enabled us to deliver more than £3.5 billion of member value, including £729 million through Nationwide Fairer Share payments.”
The building society said it recorded a profit of £959m, down from £1.3bn the previous year.
It said this was due to fluctuations in the bank’s base interest rate over the year, and its decision to “return more value” to its customers.
The firm’s mortgage balances rose to a record £210.8bn while net debt also rose to £6.3bn.
Members’ deposit balances also increased by £8.3bn to £201.7bn.
The firm also posted its highest level of lending to first-time buyers and recently expanded its Helping Hand product, enabling mortgage loans of up to six times income.