We have two cats, one was 5lbs overweight and the other 1-2. The vet said it was b/c (1) I was leaving the dry food out in a cat food dispenser all the time, not measuring it and (2) b/c dry cat food isn't the right food for them. His opinion is that cats weren't meant to eat grains (wild and crazy theory, looking at those teeth ;) ) and that dry cat food isn't good for them. When I looked at numbers, the wet food didn't seem much better, just more expensive, so we still use dry food which isn't ideal. I've known people who switched to giving their cats a mouse a day and they raved about the positive effects but I can't quite bring myself to do that.
The vet told us to count calories and we did. It didn't work very well. They ate less than the number of calories he recommended and when I went to feed them in the morning there was a lot of food from the day before. I now feed the two of them the correct amount of food for the large cat at his healthy weight. If the chubby one is getting thinner, I really can't tell. I don't think he's getting any fatter at least.
I find my vet's explanation of obesity in cats far more reasonable than the statements above about pet owners. People used to buy cheap cuts of meat for their pets (or have them catch mice). People didn't used to have this weird dry food out all the time for their cats. They certainly didn't have food out all the time in unlimited quantities. I don't think these changes are good for the cats but it is more laziness and lack of knowledge than some bizarre form of overindulgence or self medication or lack of exercise by the owner.