When you have jobs, neither employer knows about the other from a payroll perspective. Therefore each assumes that they are the only money you make for your withholding. The US has a tax bracket system where you pay higher rates of taxes on buckets of income.

The first $10,275 of income is taxed at 10%. So your second job was withheld at 10%. However you really made more than that, so it should have been taxed at a higher rate.

If your first job was say $50,000, then that extra 7500 should have been taxed at 22% (based on 2022 rates). That's why your refund went down. You earned income that didn't have enough withheld, so you have to make up the difference.

Both employers withheld correctly as far as they knew, if you put down the standard 1 exemption withholding on both w-4s. You have to follow an alternative process for second job withholdings.