Why do i always pig out hard when i have been drinking?

Alcohol: Alcohol is mild appetite stimulant, so often times people feel hungrier when they've had a bit to drink (think aperitif). Drinking also alters our judgment and you may not be fully aware of how much you're eating.

Appetite: Alcohol intake can lower your blood sugar which is a tremendous stimulant to appetite. This is more likely on an empty stomach. If you eat a light snack before you go out, preferably not carbs but some protein & fat (digests slowly) this may be less of a problem.