I am also in the same boat, having lost a considerable amount of weight in a relatively short period of time (100+ pounds in 10 months). I created a website and posted updated pictures with my weight every month and in the beginning my friends were extremely supportive.
That said, as I got "skinnier" some friends started expressing concern and told me to stop losing weight, which is something I never thought I would ever hear! lol
After thinking about it, I came to the conclusion that some people just expected me to be "stocky" (I was over 250 for ~20 years, going over 300 for the last 5 years) and didn't know how to handle the change because they had always known me that way. Even my wife had never seen me below 245 which was the weight I was able to get to when we got married. I also think that there was genuine concern for my well being and worried I was overdoing it. I was training for a marathon and was being monitored by a doctor but that didn't seem to satisfy some of them which kind of surprised me. Fortunately they are now used to the new me and think of me in the present tense/size.
Jealously did come into it in at least one case though. One of my friends was also overweight, albeit not as obese as I was. Over a period of ~3 months I noticed that he hadn't replied to my emails or messages I had left which I thought was kind of strange. Mutual friends told me that he was upset that I was able to do what he couldn't and I think my weight loss was a reminder that he had failed in his attempts to lose weight. I honestly believe that he would be happy if I put the weight back on. Sadly, I can empathize with him as I also had a friend of a friend who lost quite a bit of weight and I remember being very jealous of his success.
The final comment about losing weight really baffled me as it came from my father about a year ago. Long story short, we got into a really small "argument" about nothing significant and out of the blue he said "I think I like the fat Roger better" which really shocked me, especially since he is a doctor and always wanted me to lose weight. Very strange but he is also overweight and is getting old (83) so maybe that was the root of it.
The bottom line is that the compliments and benefits have overwhelmingly outweighed (no pun intended!) the detractions so keep at it.