I’ve had a few, particularly since having DD. I’ve always been fairly slim (but not skinny) I am also just under 5’2. So if I was a size 16+ I’d probably be rather unhealthy.
For further reference I am a size 10, was a size 8 prior to having my dd.
Comments such as
“God, how are you SO SKINNY?”
“You should eat more!” Are commonplace. I’ve also had the rather unhurtful and untrue “ah well some men like more meat only dogs go for bones.” Aimed at me, I say untrue because rightly or wrongly my bones do not stick out 
“Do you want some of my food?” - complete with a head tilt and raised brow.
And referring to me “She is such a skinny thing, I wonder how she managed to give birth without snapping”

I’ve just finished a night shift with three others who spent part of the shift making comments about my size, may be relevant but they’re all plus sized. I’ve never made a comment as I’d consider it rude and unkind.
Aibu to think it’s not ok to comment on someone’s body size either way and if it is ok, why?