I love it when someone says hey have you lost weight, you look good, but only if
(a) I have actually lost weight - some people try to convince me that I've lost weight even though I know I haven't and my clothes don't fit any differently
(b) it's not in front of a bunch of strangers - I suddenly feel like they are looking at me and judging my weight.
But, these comments normally come from my friends who I know would not treat me any differently were I slim. I hate it when people judge based on appearance or treat you differently. When I was slim for a minute, guys would be all gentlemanly, nowadays they cut me off to get on the train first or so they can let some other attractive girl get on first or almost run me over at pedestrian crossings.
Oh and it's more likely in my family that they'll talk about how much weight you've put on rather than how much you've lost.