Filing Taxes Single Verus Married?

I currently live in Texas and I am making close to $3000 per pay check (bi weekly) with all taxes and medicare taken out. But when I asked my employer to see what my pay check will look like if I was to file single with two exemptions, they said it will be $2600 per pay check. Why is there a $400 dollar difference per pay check? Is there anything I can do to increase my take home pay? Are singles in a higher tax bracket than married people? If I file as married, I will be am filing as head of household with about 4 dependents (wife and kids).