In addition to what Brick notes - particularly, the 15.3% that goes to FICA/Medicare - if you're in the 25% bracket before your 1099 income hits, so > $75,600 this year for Married Filing Jointly, you're also in the area where you start losing deductions and credits.
For example, if your combined income were $95,000 before your 1099, and then you earned $20,000 in 1099 income, you're now at $115,000, which is $5000 over the phaseout for the Child Tax Credit begins. You would have a reduction of .05*5000=$250 to your Child Tax Credit (and thus to your refund). You're now at 41.5% tax on that amount - and if your pre-1099 income was $110,000 on the dot, it would be 45.3%, based solely on this.
Other tax credits or deductions have income limits as well and may reduce your ability to take them; the IRA deduction, many education credits, and AMT exemptions all fall in the 25% bracket. Your marginal income tax rate may well be significantly higher than 25% around the $100k-$160k area depending on your circumstances.