In all seriousness though, the whole fat thing is getting old, and if Americans made fun of British stereotypes/European/Russian/etc they'd be pissed off, for example, if most Americans said all Chinese/Japanese/all Asians were squinty eyed bimbos who worked at rice fields wearing those hats shouting "ching chong" all day, they'd get pissed too, or if all British people were bad teethed sarcastic assholes who spoke so badly nobody could hear what they're saying and they'd sell their family for a little pack of tea, they'd be mad too
The only difference is that a rice-field-ching-chong stereotype or a crooked-teeth-brit stereotype isn't as prominent as the obesity situation in the states (even though from what I remember it's serious in the UK as well).
Americans are stereotyped as fat, clueless, uneducated half-wits because they are. I'm from the hemorrhoid inside the ass of the US. Southern West Virginia. Here we don't have people, we have caricatures. Not all of the US is this bad, but the majority of it is. They're over-eating, under-educated, cultureless, selfie-snapping, reality-TV-wanna-be, couch-sitting sorry excuses for human beings. And the worst part is: it's insidious. The Americanization of the industrialized world will be the downfall of modern society.
I don't, because i'm not an idiot, i know british people aren't in the 1700's anymore and chinese people aren't either, and america isn't the same as it was in 2009