You have to be careful using jealousy as a motivator. The way I see it there are 2 types of jealousy.
Benign jealousy - this is when you look at other people and say "oh I wish I was skinny too" or "darn, how did she get that job, I want to go back to school and get a good job too!" With this type of jealousy it can propel you forward as long as you don't focus on it. It can motivate you but it truly has nothing to do with the other person. Just be careful that it doesn't make you negative.
Envy - this type of jealousy is toxic. It causes you to question whether or not another person even deserves their good fortune. If you find yourself often wishing that your friend didn't lose the weight, or that she didn't get engaged or whatever it is that you're envious about then there are some serious problems here. If you start imagining that another person's life is somehow better than yours then that can be bad too, because nobody's life is perfect and size has very little to do with happiness. I've got skinny friends with all types of problems lol.
"No matter how developed you are in any other area of your life, no matter what you say you believe, no matter how sophisticated or enlightened you think you are, how you eat tells all." - Geneen Roth