Sit them all down and request that they stop talking about weight and weight loss. Tell them what you've said here: You want to live the life that you want. Tell them that pushing weight loss on you is deteriorating your relationship with them and that you will have to start distancing yourself if it continues. I'm essentially saying to give an ultimatum.

You also have to start thinking about moving out if this is really important to you because you are under your parents' roof and although you are an adult, they are in a position of power over you and you are dependent on them. The ultimatum will likely be less effective due to this (hard to avoid someone if you live in their house) and your parents' could see you distancing yourself as disrespectful and ask that you make new accommodations. So, keep that in mind.

I don't think they're coming from a place of malice, but I guess this is the advice you want.