If English isn’t your first language, it may be hard to recognise sarcasm or particular strains of humour. It may have appeared that you were being unnecessarily harsh on your self-image.
Nah. You just happened to find one person without a sense of humour. I’m almost Belgian (I’m from the south of the Netherlands) and the reasoning you suggest is just not something I’d ever consider part of my culture.
Maybe Belgians are sensitive about how skinny they are, and then you go rubbing all your luxurious American fat right in their faces!
Definitely not unheard of here, but we do prefer to deprecate the Swedes. And vice versa, I guess 
Well, I’m Belgian and I agree that this reasoning would never cross my mind.
iiandyiiii, I’m afraid your sister-in-law’s reaction makes absolutely no sense to me. The best I can guess is that she misunderstood you. Otherwise it’s… very odd.
As far as self-deprecation goes, French-speaking Belgians are pretty good at it, especially in comparison with our neighbours south of the border :D.
It may well have been peculiar to my sister-in-law; I didn’t try the same humor with anyone else. To answer some other questions, I probably am pretty typically “American-sized” (a bit overweight on an already beefy frame). Thanks for all the input!
I think this may be right, and that as a sort of Belgian Ambassador, the Sister-in-law wouldn’t want a visitor to think they’re being judged, even if they are - and the joke was too near to the truth to be funny.
Well, Belgians are practically French, and everybody knows that French lack any sense of humour. It’s a cultural thing.
This is a woosh, right? Because it’s wrong on so many levels…
Imagine you had a guest from abroad who was recently diagnosed with cancer. You and the guest happen to see a news article about high cost of cancer treatment in the US, and the guest remarks, “I guess I’m too poor to get treated in the US.” Would you laugh at this “self-deprecating joke”, or would you be horrified?
Maybe on a national archetypal level that’s true, but there are certainly a hell of a lot of self-deprecating people in the US, and good number of public figures and comedians whose personas are built on putting themselves down or playing up their flaws.
At the very least, there’s me! I’m the most self-deprecating person I know (not to boast or anything).
I think my situation was a bit different… buying shorts is not the same as paying for cancer treatment.
Do you have any examples with Nazis in them? 
I can’t give an answer on whether Europeans are commonly self-deprecating or not, but maybe your sister in-law was embarrassed. I work in a clothes shop, and while I will laugh along with those making fun of themselves for being too short, too tall or too slight for us to provide them with what they are looking for, I always feel uncomfortable when someone is too fat making the same sort of jokes.
See?
That’s NOT American-sized.
American-sized are the ones who have problems walking because of how fat they are. While we don’t think the average American looks like that, it’s a sight that’s common in the US and very rare in other places (we’re working on getting there, sadly).
My point was that in some countries, people have a much dimmer view of obesity than Americans do, akin to how Americans may feel about cancer.
Nice.
But I think you missed the “self” bit in “self-deprecation” :p.
Oh, I try very hard to use self-deprecation, but people keeps just agreeing with me in everything I say.
Just out of curiosity… Is the term ‘cheese eating surrender monkey’, a very well known phrase in France, or for that matter, anywhere else in Europe? 
I assumed it started in Scotland.