Air intake is measured by weight (pounds). The fuel/air ratio is 1:14. More pounds air in, the more pounds of fuel required. Mods to improve breathing are "efficiency" gains; more power per crankshaft revolution, so naturally, they're going to use more fuel.
If you just added your intake, the fuel injection will take about 10 trips to recalibrate.
You might see improvements by doing maintenance on the fuel injection system: if you have more than 50,000 miles, you should replace both O2 sensors. If you use a K&N, clean the MAF element (use electronic contact cleaner, any residue will ruin it). Clean out the IAC (idle air control valve at the throttle butterfly)