Why do I look 30-40 pounds lighter than I actually am?

I'm 5'4" and weigh 170 pounds, but when I went on mybodygallery.com and put up my stats, the results looked way bigger than I am. the ones that looked more like me were like the 130s and 140s. why is this? i'm so confused with the BMI and everything, cause technically I'm almost obese by my weight, but i don't look obese. people never guess my weight right, and guess in the 120s-140s IRL. i mean i don't get how i can have THAT much muscle, you know. the most I exercise is once a day, for 30 minutes, 5 days a week (which I need to increase.. as well as eat better, cause I want to be thin like I was when I was little :/)