Why do some parents call their kid fat?

My mom ALWAYS makes horrible comments about me like "your legs look thicker", "your thighs are getting so big", "OMG you're arms are huge now", "when did you gain so much weight?!" etc. My other relatives act like that too. My aunt who haven't seen me since last year finally saw me again and the first thing she said? "Oh well you've got so much bigger!" I used to have an eating disorder in 7th grade, but I hid it from everyone. I started getting better ths year and now that I'm in grade 9, I still struggle with terrible body image but I'm back to a healthy weight. I started doing more sports and eating more, but gaining more muscle and weight. My mom would tell me, in a disappointed voice, "all those running must be why you're legs got so huge" and "those sports are making you so heavy", etc. I don't understand. Why do some parents call their kids fat and think its ok? Why is it ok for them to say things like that and not understand how HURTFUL it is? How can I even reply to those comments?