The portfolio decreased to $823.4 billion, for an annualized 5.6 percent increase year to date, the McLean, Virginia-based company said in its monthly volume summary.
In May 2008, the portfolio was $770.4 billion.
Delinquencies, which increase stress on the company's capital, jumped to 2.62 percent of its book of business in May from 2.44 percent in April and 0.86 percent in May 2008.
The multifamily delinquency rate also rose, up 0.02 percentage point to 0.12 percent in May. A year earlier it was 0.03 percent.
Freddie Mac said refinance-loan purchase volume was $40.3 billion in May, down from April's $43.3 billion. March's $52 billion was its largest refinance month since 2003.
The amount of mortgage-related investments portfolio mortgage purchase and sale agreements entered into in May totaled $5.3 billion, up from April's $956 million.
The company's total mortgage portfolio decreased at a 1.6 percent annualized rate in May to $2.228 trillion.
In early September 2008, the U.S. government seized control of Freddie Mac and its larger sibling, Fannie Mae
The government is now relying heavily on Fannie Mae and Freddie Mac in its efforts to stimulate the U.S. housing market, which is in the midst of its worst downturn since the Great Depression, by buying more mortgage loans, easing refinancing and helping homeowners avoid foreclosure.
(Reporting by Julie Haviv; Editing by James Dalgleish)