You wouldn't want people to say 'You've put on weight!' would you?
So why expect people to comment that you've lost it?
There are too many people of all sizes that have EDs in varying phases and if not that, dysfunctional eating or issues around body image. It's far, far better to say nothing at all than to risk causing harm to them.
Having had an ED myself and with a DP whose own one is very easily triggered, I found that even being in the same room as somebody commenting upon another person's size was difficult - if they were being complimented on losing weight, I'd feel like crap because I hadn't been strong like them or that I'd lost more than they had but obviously still looked so fucking immense that nobody had noticed, if it was 'looking well', I'd be thinking 'spiteful bastard, wonder what you're saying about me behind my back if you say that to their face?', etc, etc.
There's so much harm that can be caused by commenting upon a person's weight - and not just to the person concerned - that it's better for the subject to not come up at all. Far better to compliment upon a particular top, earrings, hair, etc, as even if it looks great because they've lost or put on weight, saying 'that colour really suits you' or 'they're lovely', isn't publicly passing judgement upon their breasts, stomach or backside.