Piggyback financing remains a key component of the mortgage market – and a thorn in the side of mortgage insurers. But there are indications that may start to turn this year, as lenders and analysts express growing concerns about the risks afforded with the piggyback structure. Just how big is the piggyback market? Numbers are hard to come by, as many lenders portfolio the products. About $26.63 billion of closed-end second mortgages were securitized…