You're more then your pants size

So the pants on the bottom I've had for a couple years now. I havent worn them much but did yesterday they are a size 00. The pants on the bottom a friend gave me yesterday. She had bought them in her size, a size 3, and she couldn't even get them up her leg so she gave them to me. They looked small but I figured I'm normally a 0/1 so maybe they would fit. I couldn't get them up my legs either!!! Do you know who they did fit? My daughter who is in a kids size 12! Why is there such a huge difference in womens sizing? I know it can be so hard sometimes when you see your pants size go up but in reality those numbers dont mean much.