It was withholding like from your paychecks. It is not the actual tax. The 401K distribution adds to your other income and may have pushed you into a higher tax bracket. You pay tax on your total income minus the Standard Deduction (or Itemized Deductions).
And by increasing your income you may have lost some credits like the EIC . You can lose like up to 50% of it for taxes and penalties for federal and state.
As I said above make sure you are getting credit for the withholding in box 4 on your tax return line 25b for federal.
