I think Worra (who in my head is absolutely Worrals by the way) makes a very good point: people are hostile towards others for many reasons not just weight.
It does seem that unpleasantness towards fat people is perhaps seen as more widely accepted than other forms of discrimination. (Ech, am to tired to be rephrasing that. It was not meant as A Hilarious Pun though, I promise.) I think there is a certain mingling of fear ("That Could Be ME") & peculiar-misplaced assumption of a kind of moral superiority ("that would never be me, how can they let themselves get like that, they are obviously very greedy & lazy"). I also agree with AgentZigag's point about the government effectively legitimising the vilification of overweight & obese people. That said, you have probably also become more aware of comments like this as you gained weight as you now feel they are directed towards you. I mean, obviously some of them v sadly are, but am meaning you'll pick up on the background noise (as it were) of them much more.
MammaTJ & BigSpork With regards to the behaviour of medical professionals I think that doctors get so used to horses (the majority of the time for the majority of people [certain] physical health problems will be due to/aggravated by their being overweight) they forget that they need to watch out for zebras all the same. I know there is also often a feeling of frustration (similar to that around treating smokers for pulmonary problems) around patients who could improve their physical health "just" (inverted commas because while in theory it is a v simple "eat less + move more = reduction in weight" equation it's not really that easy for most people to do) by losing weight. Treating patients who are in denial about their weight can also sometimes make them impatient with patients who accept that are overweight/obese, I think. When so many people refuse to engage with the idea that their symptoms are due to their weight it must make it harder to pick out the people who aren't just in denial, iyswim? Doesn't make it any easier/better for people who have to deal with this kind of response from medics though, I know.
Also, I have frequently had physical health problems attributed to my being underweight. In some cases my weight was a contributing factor - e.g. I was on the borderline of full-blown osteoporosis a couple of years ago. Partly that was down to my being underweight (due to anorexia nervosa) but a lot of it was thanks to massive amounts of oral steroids (for my severe brittle asthma) having done a good job as stripping my spine & having a disability that impacts on my ability to weight-bear. The phrase "If you just ATE more & did a bit less exercise..." makes me want to weep because there is no "just" about it. Also, for me, the multiple physical disabilities I have mean that for me weightgain won't mean a massive improvement in my physical health. Meh. At the other end of the scale, I have frequently encountered the attitude that because I am Young And Thin I must be in bounding good health. And presumably just at the hospital to socialise 
Society as a whole does seem quite ready to jump on thin people, too. Even at my current weight (am almost in the healthy range, which in my head is The Hippo Range, though only for me, not anyone else, I hasten to add!) I still get stares/whispers/snark & people feeling free to comment on my size/apparel/shopping basket contents. The latter is a whole other world of awful because food shopping is such a complete nightmare without other people making comments like "no WONDER she's that size if she eats like that!" because I understand that to mean "of course she is a thundering great lump she eats like a whole herd of pigs". Getting "oh aren't you GOOD" for a basket of low-calorie/fat/salt/sugar/taste foods isn't awfully helpful either because it sounds like "yes, yes, eating a few hundred calories a day is GOOD, the less food the better, look at how other people think this is Right And Proper". Am not holding other people responsible for My Crazy, obviously, but I don't think anyone should have to put up with this kind of scrutiny & comment from others. Also, just as overweight/obese people are so often categorised as Lazy And Greedy, thin people often find themselves labelled negatively as people project all sorts of attitudes onto them. I'm sure some thin people ARE unbearably smug & superior, but that's only true in the same way some fat people ARE greedy & lazy.
If you are thin, any complaint you make about things to do with your size - eg having trouble finding clothes to fit - is taken to be a stealth boast & summarily dismissed. Random people feel completely free to tell you you should gain weight. People are fascinated by your diet & exercise regime. I think both ends of the scale suffer because people know that weight is a spectrum & whilst they could find themselves anywhere on it they don't want to think that. It seems people often want to believe that they are not "skinny" through circumstances outside their control yet they are not "fat" owing to their behaviours. Am sure this ties in to that trend for people to be hostile towards people who are losing weight. (I can understand being hostile towards Diet Bores, however. They make me stabby. Along of wannarexics & fauxlimics, though in a different way, obviously.) It does grate on me when people tell me I'm "lucky" to be "naturally skinny". Because there is nothing Natural about it: my set weight would give me a BMI of almost 22 & Medical Stuff means that I really "should" be overweight. It's not luck, it's self-destruction. And, of course, I never feel anything other than elephantinely enormous, however thin I am. 
Oh dear, have gone terribly rambly. Insomnia + internet access can be dangerous. Basically, yes, I do think that, whilst any (perceived) Difference can be used as an excuse to behave badly towards people, hostility towards fat people is seen as acceptable by much of society. That being said, skinny people do catch it too, because size&weight are such a fixation for humans. Gah.