"But Rio, fat people smell bad!"
I would not even dignify this with a response, except that I have heard this nonsese far too often to let it go.
First, I want to say that I have an extremely sensitive sense of smell. This is a blessing and a curse, depending on the situation, but it allows me to create exquisitely spiced dishes, among other great things. One of the curses of this sense of smell is that people with unpleasant odors are often more detectable to me than to those around me. So, let me give you my PERSONAL experiences with people and odor.
Looking back on my friends and acquaintances, I can honestly say that I cannot recall a smelly fat person in my circle, but I have had many, many unhygienic thin people. I'm not saying that thin people stink, but that body size does not determine odor.
S was a year ahead of me in collage. He was well over six feet tall, and probably only weighed about 120 pounds. S was very cute, and he had a huge crush on me. He might have stood a chance, but S was not acquainted with deodorant or toothpaste. Friends had tried many times to introduce him to the substances, both tactfully and directly, but it just never sunk in for very long. I'll give S the benefit of the doubt and consider that he might have been allergic, but he remained single until an equally unhygienic girl of average size decided to date him. I'm glad that they found each other, but I hope I don't have to be in a room with both of them ever again.
X and Y were both attractive young men of slender, but not skinny, build. X and Y seemed okay on the surface--they used deodorant, brushed their teeth, but when the clothes came off, intimacy revealed that both had some kind of aversion to washing their nether regions, especially in the rear. Despite being tactful, and eventually direct, neither would ever start washing their ass cracks during my time dating them. I mean, honestly, guys, dingleberries are for furry animals, not humans.
M was a young woman of average size. Because her parents never had decent hygiene (and her father was missing most of his teeth by the age of 50), she didn't know any better. Her hair was always unwashed, hanging in greasy clumps. Her clothing was never washed either.
C was a very tall, thin girl with whom I went to university years ago. C was very sweet, but she had a tendency to wear very short skirts, and she never washed her nether regions. She smelled like a walking yeast infection, and so did her whole room. It was really hard to visit her sometimes, because the smell gave me a headache, but I wasn't really prepared to talk to her about it.
T is a 350-pound woman. She take a shower, using soap on all of her parts. She uses a long-handled scrubber to get all the nooks and crannies. She washes her hair, brushes her teeth, and does her laundry regularly. At the end of a long workday where she has done physical labor, she gets onto the bus to go home. Because she's worked all day, she's somewhat grimy and sweaty, just like the lean guy sitting next to her. They both have a little bit of body odor, but because they are looking for the fat woman to be smelly, other people on the bus only notice that she smells a bit, shutting out the rest of the laborers whose bodies they aren't conditioned to think of as "disgusting".
This is a perception issue, one that does not resemble reality. Hygiene can be neglected in all types of people, especially those who did not have as much education (in M's case), those who had weird religious aversions to touching themselves in certain places (X and Y), and people who don't have their medical problems properly treated (C, for example). In some cases, there are people who cannot help their odors, either because of a medical condition that makes odors more prevalent, or because they have allergies to things like deodorant and certain soaps. There are also underprivileged people who do not have access to laundry facilities, a change of clothing, or shower facilities, especially if they are homeless.
So what I'm saying is, body size has nothing to do with a person's odor, and a person's odor does not determine their value as a human being anyway. If you're on public transportation, going home after a work day, everyone is more likely to stink a bit, because they have been working and sweating all day. That's life.