Increasing your 401k deduction will LOWER/DECREASE your take-home pay and LOWER/DECREASE your federal taxes in the current year.
401k is a specific amount of money that will be deducted from your initial pay so it could be allocated to your pension account.
When 401k deduction increased, it will take a bigger chunk on your salary, so your take-home pay will be reduced. The amount of 401k deduction is considered as pre-tax, so it would not be counted in federal tax.