Easily drunk?

I feel so pathetic because every time it's a holiday or special occasion I get pretty damn tipsy from even just half a glass of wine. Can anyone tell me why my tolerance is so low? I'm 18 and 5'7 and weigh like 110 lbs but there's girls even smaller than me who I've seen inhale straight up vodka and be fine and I just don't understand why I get tipsy so easy