Last year my tax return was bigger but in 2020, when I finally became full time, it's 90% lower. Why is that?

[ColeenD3]

The more you earn the more you are taxed. The only way to know for sure is to compare your 2019 and 2020 tax returns line by line.

[VolvoGirl]

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/...