Some myths about being overweight have a basis in truth and some are merely prejudice in action. One of the ones that has somewhat of a factual backing is that fat people smell bad. Of course, it's not really the fact that being fat makes you stink, but rather that there are particular hygiene issues and people don't talk about them.
One of the main issues that people need to deal with is the overlapping of folds of skin and how that causes biological interactions that thinner people don't have to deal with. Moisture-related issues and less than attentive care to cleaning and especially drying after a shower or bath are a big part of the problem. Many women with large breasts, for instance, develop yeast-related rashes and odors under their breasts, especially in the summer.
Frankly, there are also issues related to going to the bathroom and people having problems cleaning or wiping themselves properly as a result of their size. There was (is?) a device created for people with handicaps or weight issues which allows them to put a piece of toilet paper on a stick to reach their behinds after defecating. If you're overweight and don't apply extra care in the toilet, there can be odor issues as well.
There's also the point of perspiration and related odors. Overweight people supposedly sweat more than people of average weight because they have to make more effort and heat up more rapidly due to excess insulation. Personally, that has not been my experience as I sweat very little. I've read that obesity can actually make sweating less likely because the extra padding impedes the sweat glands ability to bring moisture to the surface of your skin.
All of that being said, anyone who is not attentive to the peculiarities of their own body chemistry or problems can smell bad. It's not really about being fat as it is about being less than careful about hygiene. I think overweight people may give up sometimes or not fully understand that their problems do not have to be inevitable. For instance, baby powder between any skin folds helps a lot with moisture and yeast problems. Also, being very careful to dry your body thoroughly and to clean every crevice eliminates many problems.
Also, frankly, overweight people sometimes believe that they smell in ways that thin people do not, when the truth is that everyone can have the same issue. There was a discussion on a popular weight loss forum about belly button odors and many women seemed to believe it only happened to overweight people. The truth is that it happens to everyone.
When I was around 12 years old, a good friend of mine and her sister had an exchange about this fact. My friend told her sister (both she and her sister were of normal weight) to put her finger in her belly button and then to smell it. She told her that it would smell like her (foul word for vagina). So, not only does it happen to people of normal weight, but also to relatively young people as well.
People don't talk about these things much though because it's considered too gross. There are web sites and products designed to help people of average weight handle particular hygiene problems like foot odors, bad breath, etc. (problems I have never had), but very little in the way of instruction or targeted products for handling issues related to having excess weight. The bottom line though is that being fat does make you smell bad; poor hygiene makes you smell bad and that can happen to anyone of any weight.