The way HFTs use leverage is opaque to most margin systems. Intraday margins are allowed to go way outside of overnight margins, and only positions are margined

The way HFTs use leverage is opaque to most margin systems. Intraday margins are allowed to go way outside of overnight margins, and only positions are margined, not orders. That said, there's also traditional leverage used in many cases because you may have a fully hedged position across different exchanges that don't recognize each other's instruments in margin calculation. For instance, you can be long Jan 11 Brent Crude Oil on CME and be short just as much on ICE. The risk is practically zero, so firms borrow as much money as they can to put on as much position as they can. If you've got more margin than others in these sorts of trades, you win huge when everyone else is backed up to the wall.