How do people lose incredible amounts of weight!?

wupower

I've been 5'2" and fluctuating around 140-145 for most of my 20's. I'm 28 right now.
I've always wanted to lose 10 or 20 but the farthest I've gotten is maybe 8, and that's just
from food poisoning.

Puzzling thing is, I eat VERY healthy; mostly veggies and fruit, whole grains, lean meats. I don't even
eat meat that much. I drink soymilk and eat organic, and step foot into McDonald's maybe 3 times a year?
I also exercise somewhat regularly too, gym maybe 2x a week at least and I walk to and from work and
play 2 hours of vball on Sundays. Even when we eat out (1-2x a week on average) I usually am the one who
eats half or a third of the food and saves the rest for other meals.

What am I doing wrong?!!! Why am I still fat?!
How do people lose incredible amounts of weight like 50 lbs and stuff?!!!