There are a lot of apps to help you track your weight and BMI...
But they all have a few fixed indicators integrated, and you have to live with them.
If you want to track additional things,
such as fat percentage, muscle mass, water percentage, circumference of your biceps, belly, waist, thigh, etc...,
then you might want to try the Dynamic Body Tracker! As the name suggests, it's dynamic, which means you can specify what to track yourself
Give it a try, there's a free version and a full version.
Let me know what you think