General Patreus gave his mistress 8 highly classified books with the full intention that she read them, and use them as a basis for writing a book (about him). The Justice Department called the information in those "exceptionally grave damage", and was looking at charging him under the Espionage Act. Additionally he lied to the FBI during the investigation (a grave offense in and of itself). Instead of going after him they negotiated that down to $100,000 fine and two years probation, and separately he was drummed out of the military. The final charge was "mishandling classified material"...
So not only was his offense of far larger scale, but it was deliberately giving someone classified material for personal gain, a deliberate offense rather than one of lack of care. Even the most hyperbolic accusations of Hillary Clinton come nowhere near that.
You can argue that someone with a lesser position than Secretary Clinton might have been disciplined for this, but both the FBI and Justice Department have said that there is not enough evidence for charging under one statue that requires malice, or another that has only ever been used in treason cases.
What does smack of a two-tiered system is that both her predecessors in that office, and other contemporary cabinet members, had similar email setups and no-one is talking about trying to prosecute them. Nor are we talking about bringing charges in the Bush email scandal, where the law was clearly violated and 22 million governmental records were lost... including a number that were subpoenaed in investigations of the Bush White House. If we are going to hold people to this standard, why are we not spending $20 million dollars (estimated FBI costs of Clinton probe) investigating those?