The TCJA was specifically designed to take out less tax from the taxpayer's paychecks throughout the year, thus resulting in smaller refunds come tax time.
Quick tip on how to truly compare your 2018 tax to 2017. Look at line 10 of your 2018 Form 1040 and line 63 of your 2017 Form 1040. These lines represent your total tax liability for each of the tax years. These figures are your total tax before taking into account any tax payments made such as withholding or estimated payments. Comparing these two figures will tell you if you actually paid more or less tax in 2018 than you did in 2017.
Many taxpayers are incorrectly using the amount of their refund or balance due with their 2018 tax return as a gauge of how they were affected by the 2018 Tax cuts and Jobs Act. Because withholding, estimated tax payments and other factors play into your refund/balance, this isn't a reliable indicator to determine how you were affected by the TCJA. Use your total tax instead!