Wow. I clicked on this thread expecting to be 100% on your side, @Prickleitch, but you were bang out of order.
Your husband should have been more sensitive and supportive of your weight loss, and he should have kept his mouth shut to begin with, but you did ask if he still found you attractive and he was honest.
Most men when asked by their wife if they find them attractive (or more commonly, how do I look?) know what they are going to say before they even look, because they will say what their wife wants to hear. But your DH was honest. And he did not say he did not find you attractive. He said that he did not find you unattractive, but that you were sexier when you were a bit bigger.
You could have asked for more clarity. Maybe he agrees that at 16 stone you were too heavy and it was putting your health at risk, but perhaps when you were back at 12 stone that was perfect in his eye, and now you are losing more weight and he doesn't know where this will end. He loved you with your curves and he's worried you're going to turn into a stick insect. What he said was annoying because you are rightly proud of your achievement. In your eyes your weight gain probably represented a lack of control and self-discipline, whereas your weight-loss is you exercising your choice to take control of your body. You've done brilliantly. But he likes curvy girls and he wasn't being sensitive about what was going on in your head, and as your life partner he should be.
His comment about you looking old was silly and insensitive, and probably just repeating what he hears others say. I think when weight loss becomes extreme, and someone's face looks sunken and their neck is gaunt then that is aging. But as PPs have said, an obese face sat on an obese body is also aging. For most people in the middle in my opinion losing a bit of weight makes them look younger if anything. I cannot comment on your case because I haven't seen you, but I suspect he was just looking for something else to support his argument, and I suspect he regrets reaching for that option.
But what you said, wow, that was just mean. Your DH was being insensitive but he was not deliberately cruel. You have changed your body shape and he was telling you that he likes some versions of your body more than others. You just insulted a part of him that has never changed. He told you that he found you beautiful and sexy as a curvy woman, which is what you were when you met and what you have been for most of your lives together. You told him that he always had a small dick and you always wished it was bigger - when you met, today and every day in between.
I think he needs to apologise and be more sensitive, but if either of you said something your relationship may never fully recover from, it was you @Prickleitch.