Yes. The biggest part of that refund is the Earned Income Credit (EIC). The basic EIC works on a "bell curve,"
rising as a worker's wages rise reaching a maximum when annual earnings are
between $13,650 and $17850 (Single with 2 or 3 children 2014) and then
declining gradually until it phases out altogether. If your income is on the up
slope of the EIC curve more earned income will increase your EIC but if your
income is on the down slope, more income (of any kind) will reduce you EIC.
http://www.taxpolicycenter.org/briefing-book/key-elements/family/eitc.cfm
