A cut in the size of mortgages that Fannie Mae and Freddie Mac can finance will likely increase banks’ holdings of so-called jumbo loans rather than boost sales of private home-loan securities, a Wells Fargo & Co. executive said. About $30 billion to $50 billion of additional mortgages that are larger than the government-supported mortgage-finance companies can buy would be written annually if the reduction is put in place, said Anthony “Tuck” Reed, senior vice president of capital markets at Wells Fargo’s mortgage unit.