My best friend (female) feels the need to compete with everything I do?

Hi, so I'm 23 and my "best friend" is 24, we have been close since we met when I started a new school at 8 years old. Even back then she seemed to have jealousies, I was a state competitive runner and she was a state competitive swimmer but she still had a hard time dealing with the fact that she would always come second to me in a running competition. As we got older she seemed to have insecurities about her body compared to mine, I was a very petite girl with long blonde hair all the way down my back and a couple of inches taller whilst she was shorter, chubby with short, thick, frizzy hair. In my teens She would rip me off about my small breasts and slim figure and constantly compare our bodies to one another around boys. If we go shopping- like anywhere she almost runs into the store and has to look at everything first so she has first choice. I could be about to buy something and she will take it and buy it for herself. Now, She has grown her hair as long as mine and she's talking about dying it blonde. I gained some weight about 2.5 yrs ago which made me slightly larger then her. I told her recently that I was going to try and shift some weight to try and he healthy and she admitted that she "couldn't stand the thought of me being smaller then her" so she is now practically starving herself of food and nutrients, she asked me if she could copy my favourite Pilates DVD (which I had been using to assist with toning up my muscles daily) I let her, and she has been doing the excersise DVD 2-3 times a day. Why is it that my best friend feels the need to out do me with every single thing I do?