Historically, jumbo-mortgage rates have stayed about 0.25 to 0.5 percentage points more than the rate on conforming loans, an article in the San Francisco Chronicle's SFGate said.

However, the rate difference between the two has become increasingly smaller, thanks to a variety of factors.

They include rising guarantee fees on conforming loans, which make them more expensive relative to jumbos; fears that the Federal Reserve will soon scale back its purchases of mortgages backed by Fannie Mae and Freddie Mac; and a growing appetite on the part of banks and investors for jumbos and jumbo-backed mortgages.