Milk Fat content

an

For my friends who have lost all the weight and then some from nursing I have noticed their breast milk has a LOT of fat in it. My milk in a 5oz bottle typically has a thin little layer. Its not that I dont eat enough, or have little fat to lose so I dont understand why my milk is so low in fat and I DONT lose weight nursing! Can someone give me a science lesson or just vent with me.