Shrinking credit card balances are forcing issuers to sweeten their balance transfer deals as they try to generate some growth in their card portfolios.According to Mozo's latest Banking Insights Bulletin, in the past month Westpac has extended the zero interest balance transfer offer on its Low Rate Card from 12 to 14 months and NAB has launched a 15-month zero interest balance transfer deal in its Premium Card. NAB has also added 12-month zero rate deal on purchases for new Low Rate cardholders.Citibank launched a stripped-down card called Citi Simplicity, offering zero interest on balance transfers for 12 months.Mozo said that ANZ was maintaining a campaign it launched a few months ago, offering zero interest for 16 months on its Low Rate, Low Rate Platinum, First and Platinum cards.Issuers are keen to build up their credit card portfolios because arrears rates are low and margins are high. However, business has been slow.According to the latest Australian Prudential Regulation Authority banking statistics, issuers' credit card balances grew by just 1.2 per cent over the 12 months to August and fell by 1.7 per cent over the past six months.All issuers surveyed by APRA are showing outflows over the past six months. Macquarie Bank doubled the size of its book to A$665 million when it took over the Woolworths credit card account from HSBC in May. Since then the value of its book has fallen to $645 million.Earlier this year, Mozo reported that zero interest balance transfer deals were becoming longer and more prevalent, with a record number of zero interest offers hitting the market in February. It said that before this year most zero rate balance transfer offers were for six months or less, but now offers running for more than six months were more common.Mozo research director Andrew Duncanson said he expected to see a 24-month offer in the near future.