No, not at all unreasonable, I absolutely think it's rude - and weird! I can't imagine doing it.
I do have some personal experience, but can't really say it's about 'negging' or involves people trying to put me down, I'm really not sure what motivates it, although the people who do it are friends, family, etc.
I have had one asshole approach me in a bar and say 'Christ! How can you function being that thin?' I promptly returned the compliment regarding his baldness (which I don't actually see as a negative) and he quickly shut up.
Friends, mostly male, occasionally make comments in a jokey or sweet way, such as 'get some weight on, we need to fatten you up!' and it is never delivered in a rude way, almost sweet to be honest, but essentially it is still fucking mindblowing that anyone even brings this issue up!
I am 7stone13 and 5'4. I eat. I have anxiety. My weight goes up and down with a few pounds but never reaches an extreme. I suppose it's just normal for me.
That said, my dad used to say 'don't fall down a grid!' in jest and it would be hard to take offense.
It's a mixed thing, I wonder if people would utter their thoughts if i was overweight?
I don't know.