I'm no expert, but from what I read in the local papers the Petters sentence is a function of a few things:
it was a long running fraud, and multiple frauds, not an isolated incident
he did not plead guilty (kind of a catch-22)
many people were hurt
it's intended as a warning to others.
Perhaps if he had been sentenced before some of the larger scandals came to light (such as Madoff) he would have gotten off a bit easier. But the public is angry.