I file married filing jointly for tax return this year. My income is 3 times lower than last year. I paid estimated tax all year. Why my tax refund is so low

[MaryK4]

If you filed jointly, your income and tax is computed on the combined amount so if your spouse's income/withholding was not sufficient your estimates may have been applied to the tax owed.  Also, perhaps if you were self-employed this year but not last year, your estimates are also applied to pay the self-employment tax in addition to the income tax.