It’s amazing how many people are invested in others’ weight.
The OP doesn’t even have any context, and people are reeling off about other people they know who they think are underweight.
My answer is MYOB, you don’t know what their natural weight disposition is. You wouldn’t dare approach someone you think is overweight and express concern.
You’d assume that they already know, or are naturally heavy set.
And if they are someone with a medical condition, or an eating disorder, your feedback won’t be helpful or appreciated.