I'm not bothered about weight, but I do expect people to dress up a bit for a job interview, even if that means clean jeans and t-shirt, rather than a tailored wool suit and silk shirt. What I've worn to interviews (of which I've had a few in the last 18 months,) has been slightly smarter than I actually wesr to work, because I want to make a good impression. I would avoid wearing stained clothes at any time, but sometimes, drinks spill, birds shit on you in the steet, that sort of thing, and you haven't time to change, and the chances of that happening seem to be exponentially higher when you're about to step into an interview.
I do work that is mainly office based, but even if I didn't - I grew up on a farm, so a suit probably isn't appropriate, but overalls when you've come from being literally knee-deep in pig shit isn'tright for an interview, either - I would expect people to take the potential job seriously, and at least aim for clean clothes, shoes and brushing their hair. In most jobs, you are presenting a particular face to that world, so I don't show all the aspects of me I would show if out on a date, just as I don't go into technical details about my job role on a date. And my family get a pretty sanitised version of my dating life... we don't show all of ourselves all of the time to all people, and dressing cleanly and appropriately is part of showing you understand that. And clothing is something you can change. There are charities which help with clothing for people who are referred from the job centre.