How come there is a large tax deduction for married couples, but nothing translates to the actual refund itself?

[ThomasM]

It translates, but in a specific way.

Deductions -- regardless of the type -- lower your taxable income.  That means a $12,600 deduction for a married couple filing jointly will result in $12,600 less in income to be taxed at your tax bracket rate.  So if, say, you're in the 25% tax bracket, then that $12,600 deduction could actually save you $3150 (12,600 x .25).  If you're in a higher bracket, your actual savings will be greater; in a lower bracket, less.  That's how deductions work.

In contrast, a credit (not a deduction) is applied after your tax owed has already been calculated, and then it reduces that tax dollar-for-dollar.  That's why, generally speaking, credits can have more impact than deductions.

So that $12,600 tax break for a married couple is a nice thing, and it's lowering the amount you owe, but it gets applied in a specific way and is not the same as a credit.