Your Standard Deduction (and personal exemptions in 2017) did not double.
If you made $18,000 in 2017 and claimed a $10,000 for the Standard Deduction and personal exemption, your income subject to tax was $8,000.
If you made $36,000 in 2018 and claimed a $12,000 Standard Deduction (there are no personal exemptions in 2018), your income subject to tax is $24,000.
In that example, even though your income doubled, the amount of income subject to tax (after the Standard Deduction) tripled.
Does that make sense?
