You may not have done anything wrong. The only way to know for sure why your bottom line is different is to compare the 2 tax returns. That's what you'll need to do.
Here are a few things to focus on:
Start with line 22 of the 1040 (line 15 of 1040A). That's your total (gross) income figure. If this figure is higher than last year, then you're starting the return with more income to pay tax on.
Next, look at line 36 (line 20 of 1040A) which will show the total amount of adjustments (subtractions from income) for the year. A lower figure in 2016 will mean there's less being subtracted than the previous year. Line 37 (line 21 of 1040A) will then show the Adjusted Gross Income figure, and you can see how the two years compare.
On page 2, lines 40 & 42 (24 & 46 on 1040A) will show your deductions & exemptions, and line 43 (27 on 1040A) will give you the Taxable Income figure, which is what you'll pay taxes on. Line 44 (28 on 1040A) will show the actual income tax based on the tax tables.
That will give you the basic information. You can continue further to see how certain credits compared, and whether you paid any penalties. Not all lines will have amounts, so you can easily focus on the ones that do, to see how the two years compare.
That's how you'll proceed, and it will make understanding the differences between the returns much easier.
