Weight Loss Explanation?

Does anyone know why some women lose weight while pregnant? I am 5’2 and at my first OB appointment I weighed 167 which is considered overweight.. I am now about 20 weeks and I’ve lost 10.. my dr. Is not worried but I’m so curious as to why it’s happening? I have not had any sickness and I only eat sort of healthy (cravings get me every time 😩)