How very brave of this king among men to shout something he clearly was desperate for you to know, out of the window of a moving car where there's no opportunity for him to have to face any concequences of his behaviour.
Hardly someone who's opinion matters to you OP, surely if someone feels so strongly about what you look like that they need to tell you, they really should be able do it face to face and be prepared to deal with any concequences?
(I'm not advocating this approach, it could be dangerous) I once got off a bus and had had a few of these incidents in a relatively short space of time, I'd had a shit day and was just in the mood for an argument, when a car full of young lads drove by, slowing down and shouting 'Bloater' 'Fat Cunt' and other intelligent and well put together comments on me being overweight.
So I stopped and said 'Why don't you get out and come and say it to my face? You're so manly and brave surely you can manage that?!' he slowed then to a stop and so I just stood there, arms open in a 'come on then!' gesture, I may have even said it. So brave were these men that they disappeared in a roar of the engine and a very classy and cool wheelspin to show me!
Made me feel better tbh, although like I said, not something I'd recommend because I'd have been way out of my depth had they taken me up on it. It showed me though that people who do this don't ever expect to be challenged and don't have the courage to face being challenged and therefore run away instead.
Like I said, not someone who's opinion I particularly value anyway.
Well done on the weight loss, you're not doing it so wankers like him give you their approval are you? I'm doing it because I want to feel better, and be fitter and healthier, I suspect that it's the same reason you are. Men like that will always find something to pull a woman down for - scared we're better than them I think and they can't handle that.
Good luck keeping going, it's hard but I'm finding it worth the differences so far.