Usually more income = more tax = less refund. Your withholding didn't increase enough to cover the tax due. Or some credits you got are reduced because you made more income.
And the IRS changed the withholding tables so you got more in your checks during the year instead of a big refund. Or there are other tax changes. And any dependents you have are a year older. If you made more income that can reduce some credits you got like the EIC.
Here's a good FAQ on why your refund might be less than last year
https://ttlc.intuit.com/community/refunds/help/why-did-my-refund-go-down-compared-to-last-year-s/00/...
When you enter one taxable transaction, you can't just watch the monitor. You increased your overall adjusted gross income and with that come many other changes in your return, not just the incremental tax on the one transaction.
Like for example it increased your AGI and that would decrease some deductions if you itemized on Schedule A. And by increasing your AGI it might reduce some credits you were getting like EIC. And can make more of any Social Security taxable.
