Hi, sorry you are going through this, I know it must be disheartening.
I personally don't believe it is because you are overweight. It is probably down to lack of experience and having to explain a 13 year gap on your CV.
Interviews are very heavily biased towards asking questions about your work experience eg "tell me a time you had to solve a problem at work" or "when have you had to deal with conflict at works".
Employers will also be suspicious of a 13 year gap without a good explanation.
You might want to practice your interview techniques, as what you are doing now is not working. It is the little things imo, firm hand shake, asking them how their day is doing, showing a lot of enthusiasm by asking questions about the role etc.
If you have an interview for Waitrose, for example, research the type of questions they ask so you have some idea in your head.
It might also be an idea to volunteer so you can come up with answers to the interview questions, even working in a charity shop helped me get a job, as I could use examples from my time there.
Good luck.