There seems to be a lot of posters suggesting that weight is nothing to do with poor hygiene. Actually, weight can be everything to do with poor hygiene.
Different people carry their weight differently. Someone who carries a lot of abdominal fat, may have an abdominal pannus (also known as an “apron”) which is when the abdomen “overhangs” or hangs down. The person in question may not be lifting the pannus up and washing it effectively, or may not realise they can apply antiperspirant under the pannus (and also under the breasts and along the “bikini” area), they may have an extremely overhanging mons pubis and thus be unable to wash their genitalia properly, they may not know that applying glycolic acid to those same areas and leaving it on overnight (best to do this at weekends for reasons I will outline in a second) effectively kills the bacteria which lives on the skin and causes sweat to smell, and lastly, a lot people don’t realise that antiperspirant should be applied in the evening because the way it works means it has to dry completely to be effective and because antiperspirant works by superficially “plugging” the pores, thus stopping the feedback mechanism that tells your body to produce sweat in the area where the antiperspirant has been applied.
Deodorant is not the same as antiperspirant. Antiperspirant stops you sweating, deodorant makes you smell nice. Showering quickly to feel refreshed when you wake up, will not wash the antiperspirant off, unless you’re scrubbing away, which you shouldn’t need too!
So, yes, morbidly obese people may not be able to wash themselves effectively because of problems of reach, inability to lift an extremely heavy pannus, an overhanging mons pubis or some other reason! A more slender person may have a smaller pannus through childbearing and not realise they should lift the pannus because, like feet, they figure it’s not a problem.
I would add that whilst a super fit person might not have the pannus/mons pubis problem, I have worked with people that have smelt less then desirable, been slender, but just don’t realise they smell, for whatever reason.
Food can also play a huge role in how your sweat may smell, especially foods that belong to the Allium family.