Hi! Welcome, new member!
The thing is, once you give up meat and eat nuts and other protein - containing foods instead, you can still remain with your current weight, as you eat more to substitute. But, your body is just used to it, that's why. I heard from other long - term vegetarians that you don't need nearly as much protein as most people consume today. You need very little, in fact. It's just your body getting rid of old habits when you feel hunger. So what you can do is eat more fresh fruit and vegetables. They make you full without giving you hardly any calories (except several tropical ones, like grapes and bananas, but still not too much). Instead, they produce enzymes that aid digestion! Which helps break down fat, as you understand. So, you just have to be patient with that - it doesn't start working right away, takes some time. WIth me, it was around three days. But people are different. Good job being a vegetarian!!