I think there are two ways for computing taxes on bonuses.

If the bonus was {treated as if} tacked onto a regular paycheck, the dumb withholding algorithim annualizes that one paycheck as if it were standard pay for every paycheck in a full year. That would make it think your income for the year is huge and withholding should be very high.

If that's what happened, you get a refund when you do your taxes, because the actual tax on your overall income isn't as high as the withholding algorithm calculated.

You could adjust a new W-4 to force a little less withholding during rest of the year from each pay check. The steps to do are a little convoluted to describe, so you might let the online tool at www.irs.gov/w4app help you out. You can pretend you had two jobs this year (one from Jan-now that you no longer hold, and one from now-Dec), tell it the total income from the first job and total withholding (including your big check), and the anticipated income from the second job; it'll give you a suggestion about how to set up the new W-4. Probably will have you enter some amount on Step 3 as if you had a "tax credit."