Super-sized hip-hop star Christopher Rios, known as Big Pun (short for Big Punisher), died suddenly yesterday at the age of 28. The cause of death was not immediately apparent, and an autopsy is to be performed today. His publicist David Granoff, however, said that it is probable that the 500-plus-pound rap artist suffered a massive heart attack. Pun, a native of the South Bronx whose rap album “Capital Punishment” went double-platinum last year, was rushed to a White Plains, N.Y., hospital Monday afternoon after collapsing in a nearby hotel. He was living there with his wife, Liza, and their three young children while renovations were being made on the family’s apartment in the Bronx, where fans and neighbors gathered Monday night to mourn the well-regarded rapper’s death.