First, if you made $6,000 last year, you most likely had no tax liability and got back everything you paid in. This year, however, because your income increased, you now have a tax liability (the amount you pay the government based upon your income) - you are probably in the 10% tax bracket. Assuming standard deduction and 1 exemption (yourself) your taxable income is somewhere around $10,000, which means you have a tax liability of about $1,000.
If you had the same percentage withheld from your income (about 10%), last year you would have had between $500 and $600 withheld (10% of $6,000), which your would have gotten all back [by your figures they withheld a bit more so you got a bit more back; not unusual). This year, withholding the same 10% means a total of $1,800 was withheld...after deductions and exemptions, your tax liability was again about $1,000, meaning a refund around $800 (by your figures, they may have withheld a bit less, bringing you tax refund down some).
Refunds are not solely about how much you made, but also how much is withheld, how much you have in deductions, etc.