Commercial lending standards have tightened in the past year for small businesses and scuttled a major portion of contracted transactions for smaller properties, the National Association of Realtors reported Thursday.

"There have been notable improvements in capital for large commercial transactions valued at $2.5 million or higher, but there remain significant challenges for small business," said Realtors' chief economist Lawrence Yun.

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According to Real Capital Analytics, more than 13,000 major properties valued at $2.5 million or higher traded hands in 2011. Sales volume increased 51 percent over 2010 to $205.8 billion, with the largest share of lending funds coming from big banks. Other funding sources include insurance companies and institutional investors.

— Alan J. Heavens