Much has been made in recent months of Yellow Brick Road's emergence as a challenger for the role of "fifth pillar" in the Australian mortgage market.The notion was given an airing in November, when YBR signed a loan origination agreement with Macquarie Group, and has been revived this week, following the news that Macquarie has taken an 8.3 per cent stake in YBR with a A$6 million investment.The idea is that Macquarie's strong balance sheet combined with YBR's growing distribution network will be a force to rival the big banks.This is far-fetched.At September 30, Macquarie reported that it had A$30.8 billion of retail deposits, including $17.3 billion in its cash management account. Its deposits increased by eight per cent during the year to September. It had total deposits of $36.2 billion.The group has a strong balance sheet, with cash and liquid assets making up 26 per cent of funded assets. Macquarie has conceded that it has more surplus capital than it needs, and this year started a buyback program to release some of that surplus.Part of the fifth pillar argument is that, whereas once Macquarie would have put this money to work buying assets and putting them into funds, now it wants to put its money into the Australian mortgage market.Since the financial crisis, Macquarie has increased its focus on traditional banking activities. Its commercial finance businesses, which include vehicle and aircraft leasing, have been an important source of growth.Its home loan business, which shut down in 2008 and re-opened in 2010, returned to a positive net inflow position in the middle of this year. The latest Australian Prudential Regulation Authority figures value its mortgage portfolio at $11.7 billion.Macquarie is allocating more of its capital to lending than in the past, but there is a limit. Traditionally, the group has regarded lending as a low-return business and it will want to keep capital in reserve to allocate to higher-growth assets (such as funds management) as other markets recover.Currently, ING Direct is the fifth biggest mortgage lender in the Australian market, with a $37 billion book. It might be overtaken by Suncorp ($35 billion) in the coming year, because of the Queensland group's higher growth rate. Neither would classify themselves as a fifth pillar.Even if Macquarie was to double the size of its mortgage book over the next couple of years, it would just make it into the list of the top 10 mortgage lenders.As to Yellow Brick Road, it has had a home-loan origination agreement with the Gateway Credit Union since late 2009. The company's mortgage book, at June 30, was worth $865 million. YBR doubled the size of its loan book in 2011/12, but it is still very small.The company is relying on its expanding branch network to drive growth. At the end of June, it had 120 branches, with representation in every state except Tasmania.YBR's founder Mark Bouris likes to have a strong high street presence; he pursued a similar distribution strategy at Wizard. Branches are still an