There are several factors that could change the amount of your tax refund - resulting in either a larger or smaller refund than expected. Examples that could increase your refund are math errors and other mistakes. Examples that could decrease your refund include math errors, mistakes, owing delinquent federal taxes, state taxes, child support, student loans, or other delinquent federal obligations or if the Earned Income Tax Credit (EITC) portion of your refund is withheld pending further review.

For additional information, see - What if a mistake on my return increases the amount of my refund?, What if I owe back taxes to IRS?, How will IRS handle my split refund deposits if the Earned Income Tax Credit (EITC) portion of my refund is withheld pending further review? Are there other conditions that could reduce the amount of my refund and change the amount I want deposited to each account? And What will happen if I owe both back taxes to IRS and back child support, state taxes, student loans, etc?

Source: www.irs.gov