The taxes get made up on the back end.

Bonus money is taxed at the rate that applies if that was your regular salary. In other words

If you make $1,000 a week that's equivalent to $52,000/year salary. And it's taxed at that rate.

If your bonus works out to a $2,000 a week rate that is taxed as if you make $104,000/year.

However, once it is time to actually do your taxes the IRS will see you made $52,000 in salary and $2,000 in bonus. So your actual taxes owed will be on $54,000.

So whatever extra taxes you paid at bonus time get returned when you do your taxes.

I used to work entirely on commission, and occasionally I'd have such a good week I'd hit a ridiculous tax bracket. Most weeks were ass though, so tax season was always great because I'd get that money back