If that other person looks worse then the person commenting must not be that bad. I agree insecurities is sometimes the reason.
I also think some people, I kind of see it run in families, tend to comment on others in general. My family tends to comment if someone is gorgeous and also if someone has gained a lot of weight. If they have always been heavy they would likely not say anything. My husband’s family comments on how fat people are, and especially if their clothes are too tight, or worse if the person has skin showing and you can see the rolls of fat their classic line is, “do they even look at themselves?” My family would never say that. His family rarely comments on how beautiful someone is.
None of these people would do it on facebook! Except the positive things like telling a friend they look fabulous.