The average subprime MBS deal got a little bigger last year. According to data compiled by affiliated newsletter Inside MBS & ABS, in 2004 the average subprime MBS transaction weighed in at a hefty $613.9 million, up from $517.1 million in 2003 and $427.5 million in 2002. So why the increase? Continued strong issuance of net interest margin securities and a string of goliath transactions by several key issuers are the primary factors. …