Well, if I make $200k, then my payroll tax is $117,000 * 12.4% = $14,508, which is a rate of 7.25%. My wife is a housewife who makes 0, so combined we pay 7.25%.
If, on the other hand, I make $100,000 and my wife makes $100,000, then our payroll tax burden is the full 12.4%, or $24,800.
Income tax kind of works the same way for married people. If you make a similar amount of money, then you pay a lot (actually, more than you would pay individually, if you can believe that; not sure what genius came up with that one). If one spouse makes significantly more than the other, then you potentially get a large break. It's a terribly unfair system, IMO. But everything looks unfair when you're the one getting fucked.