Yeah seems you friend is jealous , could be a number of reasons
1. that you are regaining some control over your life, starting a weightloss plan isnt just about what you eat its about a life style change and it needs commitmnet motivation, and organisation. Maybe she sees she hasnt got that control on her life and struggles to accept you have. Its natural to have a bit green eyed monster now and then but the fact ahes expressing it to you means theres a real problem of competition.
2. The fact you have always been bigger , she could finally feel threatened by you? that you may become smaller than her. Maybe having you as a bigger friend has always made her feel better about her self and now that could be taken away from her.
3. maybe shes unhappy with her weight also and see s it as a competion
Either way you need to see it out with her , suggest dieting together. But if she still going to be bitter .......i wouldnt fall out over it,realise when you do meet your goal weight that no one can take it away from you and you done it all your self and other peoples catty remarks will only make you stronger . Its them who have the problem. And its you who has made a great decision
I wish you all the success with your goal