You need to compare 2019 with 2018 line by line. But watch out they changed the lines numbers so it might be tricky.
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/...
