Yes, unfortunately I think it is. As some pp's have said, being overweight comes with a stereotype of being lazy and having no self control.
Total inaccurate stereotype, without all the health issues that can lead to being overweight, I think the proportion of lazy ppl that have poor self control and are fat, is probably the same as the proportion of size 8 lazy people with poor self control.... Or indeed any size/weight.
You could say alcoholic dependent lack self control, drug addicts lack self control, smokers lack self control (and get extra breaks so are also lazy,) because those also impact on someone's health and are lifestyle led..... But that's not something that is generally on show at an interview and size is. And you can't hide being fat, you can hide alcohol/drug/nicotine dependence.
It really shouldn't make any difference unless there is a specific reason that would prevent an overweight candidate doing the job - like the pp who mentioned a lady who couldn't fit behind the counter, or where physical fitness is key to the role - a fireman for example.
Unfortunately, people will and do judge. I was rushed in to have my gall bladder removed a few years ago, my (male) boss said 'well you are fat, fair, female and 40'
Ummmm, not to mention the hereditary factors like every female between 30-40 in my family has lost their gall bladder, as a family we're prone to gall bladder disease! and I wasn't 40 you cheeky bastard! I'm the only one above a size 12 in my family, except one distant cousin.
So yes, I do think it can be a disadvantage, but it shouldn't be.