Morbid curiosity perhaps? Feeling uncomfortable perhaps?
I said yesterday that sometimes it's hierarchical self esteem. Some people get their self esteem from believing they are 'better' than others. In some cases this is because they are slimmer, in my case it was because I'd been single for a long time and that was my personal 'flaw' in my friends eyes. Eg she might be x, y, z but she's stil single - no one wants her. When I got into a relationship it upset her apple cart so to speak
My mum was a bit like this. She had very low self esteem and ditched all of her friends who were her peers and became friends with a lot of women she deemed 'beneath' her. Because she believed they looked up to her and admired her because she wasn't eg a cleaner, owned her own home rather than living in a council house/rented accommodation, was more intelligent. She also dated very far down when she dated men. Men she believed would never stray because they could never do better than her. Not because she was amazing and she was filled with self confidence but because, in all honesty, no other woman would have touched them with a barge pole. She enjoyed having people comment positively on her lifestyle rather than seeing it as nothing special.
She couldn't cope with being around people she perceived as 'better' than her and needed to be able to look down on people to feel good about herself.
She was quite an extreme example of this but it is quite common for people to feel unsettled when one of their friends 'betters' themselves in any way - weightloss, relationships, income etc
Most of us are just pleased for our friends and don't see it as any reflection on ourselves. But some people assume that others see the world through the same eyes and believe that the eg slimmer friend is now looking down on them in the same way that they previously looked down at their friend.
It doesn't really matter that the OP has only lost a stone. She is following a trajectory that the slim friend might not like. The OP has also alluded her showing this competitive streak in other areas. And, tbh, when you know people well, you just know.