For a lot of people with eating disorders any comment about their appearance and attractiveness will only fuel the eating disorder. If they get lots of compliments when they're dangerously skinny then they'll obviously conclude "this is good I need to be skinny/skinnier." However, when people say things like you did like "they were cuter before" it almost always spirals the person into trying to "fix" their appearance even more

You have to remember that unfortunately people with eating disorders do not see themselves objectively and they often are not thinking straight bc their body is struggling to survive. So even if you're trying to say something positive, that's not necessarily going to be the effect it has on the person. This is why it's important to generally center the discussion around having a healthy lifestyle rather than appearance (actually being healthy, not when people use the word "healthy" to disparage anyone bigger than a size 2)

Also additional note that isn't necessarily to do with this conversation, but people with eating disorders will often have their weight fluctuate drastically. Bc their eating habits are so inconsistent, their bodies get used to holding onto as much as they can, so it's common for people with eating disorders to rapidly gain weight and then rapidly lose it, in a cycle. And of course, when they lose the weight people will compliment them usually, thus fueling the eating disorder. This is why it's generally best to never comment on someone's weight unless you know them really well and they've specifically been talking about losing weight in a healthy way. (And even then the focus should be on the benefits of the change like "you seems so much more energetic now/you look more comfortable in yourself" rather than their weight directly)

I don't think you meant anything bad by your comment. I just think you don't know much about eating disorders so this is a good learning opportunity