I think thin-shaming has its roots in the same systemic oppression of women which insists we adhere to certain beauty standards, but that we don't consider it in this context because we see it as relevant only to fat-shaming, due to the fact that it takes a very different form. Often it doesn't even really mean 'you're too thin', but 'I felt the need to make a comment about how you're not perfect but had to couch it somehow'.
Women are asked to fit into a very narrow conception of what attractiveness is. This means that many women - overweight or underweight - will struggle to be seen as attractive by those who create and perpetuate this conception, as it is so exhaustive ('blemish-free skin, tick, hourglass figure, tick - ah, but we don't like your cankles, end of the road for you') and specific, and indeed it will be completely out of reach for many. It is impossible not to have a 'blemish' or 'defect' anywhere, so there is always a reason to punish a woman on the grounds of her appearance. That said, 'thin' is in, but 'stick thin' is vilified. Overweight is always vilified simply because it is 'not-thin'. I would suggest that, crucially, more people are seen as unacceptable because they are overweight than seen as unacceptable because they are stick thin, simply because the 'not-thin' area is so expansive and contains more variance, whereas stick thin is differentiated from thin only by a very slight margin, since the beauty ideal is already for people to be practically underweight, and significantly fewer people will fit into the very-underweight category. In this beauty climate, once you verge on the wrong side of 'underweight', there is a short way to go until you have serious health issues or die, whereas once you're past the 'acceptable' thinness and into the 'not-thin' category, you can still be healthy if you don't have the required BMI - e.g. size 12, 14 etc - and thus those who are 'not-thin' are much more visible by virtue of just being greater in number.
Consequently, anti-fat sentiment is overwhelmingly more present in, say, the media (however subtle or overt it is), and so in popular discourse, because there are more people against whom to direct it.
When people are criticised for being slim, it often isn't recognised that the person criticising is playing right into the same sort of attitude that produces this anti-fat feeling i.e. 'this woman is falling short of XYZ standards, presented as my own opinion'. However, due to the reasons outlined above, it is not seen as symptomatic of any structural oppression of women because there isn't much overt evidence of anti-thin feeling being disseminated by the influential and anonymous groups who control e.g. advertisement and marketing. Our awareness of what constitutes 'unacceptable' on the other side of the weight spectrum is much more vague, although we still operate within this understanding that only a very small space on the spectrum - somewhere very left of the centrepoint, but not too left - is 'acceptable'. It is more nebulous not only because fewer people statistically will be the 'unacceptable' version of thin, but because we use the same word, 'thin', to refer to its 'good' and 'bad' versions, whereas 'fat' is always pejorative - so you'll hear someone saying in reaction to someone's change in weight, 'don't you look lovely and slim!' but never 'you're looking lovely and fat', even if the weight gain was medically advisable or safe.
If 'thin' is then contextualised as negative, it thus comes across as a very personal conclusion, at which the person has arrived in their own individual vacuum, and not representative of a hegemonic socially and institutionally constructed attitude which much more explicitly tells you not to be 'not-thin'. A personal remark may smart, but at least you don't feel like the world is against you; you can dismiss them as rude and weird and shrug the comment off. It won't serve to consolidate other accumulated remarks you receive from family, thinly veiled criticism coming up unexpectedly on TV, the realisation that you hardly ever see an overweight model advertising a range not specifically designated as 'plus-size', comments from professionals you thought could be trusted to be sensitive...to form a homogenous mass of contempt on which you can count everyone with whom you come into contact having been exposed to and, to some extent, having absorbed. The people who make these comments are aware of this.
The fairly insidious nature of thin-shaming means that we don't recognise the overarching structure of the 'beauty myth', as we're led to think that, due to its scale and our common perception of 'thin' as a uniformly privileged sphere, it's disconnected to other issues which have begun to be addressed (e.g. fat-shaming is become less overt because it has been identified as a problem). People feel freer to say shit like this as they don't see how it links in with harmful expectations of women's bodies generally. And recipients feel freer to ignore it because they think it is connected to personal preference.
Very misunderstood. I think people who challenge thin-shaming are right to do so, despite the fact that it doesn't take the same form as fat-shaming (so that people will raise an eyebrow at the comparison), as it has no place in a society where health and not beauty is the motivator behind weight loss or gain.