It's really not true. It's been well documented that Americans log more hours each year compared to workers in other industrialized countries. For instance, we average around two weeks vacation a year, while most of Western Europe takes a month, at least.
Our problems with weight, as a nation, are a reflection of that addiction to work. Most jobs involve sitting in an office, which is sedentary. And fatty fast-foods are used in lieu of home cooking, because people don't feel like they have the time to cook. Americans work about 200 more hours each year, compared to what they did in the 1973. (So says the Economic Policy Institute and wikipedia)
The 'Americans are Lazy' meme is quite useful for politicians, and their masters, when telling people to tighten their belts and work harder - to "make sacrifices". It's also useful for foreign governments who want to trash the American population. And since our entertainment industry creates an image of America as good-times rolling, this is what much of the world thinks of us. But was 'Friends' a really accurate representation of American life ? Or an illusion to create a few good laughs?
Far from being lazy, Americans work too much, and have their labor directed in the wrong way. And have, over the last generation, had their standard of living systematically eroded so that a narrow section of the oligarchy could expand their wealth to obscene levels.