This nonsense can start toward the very young. As a toddler I was rounder than my siblings and my silly father gave me stupid nicknames, Fatty Annie, Annie Fanny, Chub, etc. Perhaps it's a blessing I can no longer remember them all at this moment.
Finally around age 10 or 11 I complained to my mom and she made me go tell my dad (then age 31-32) that it hurt my feelings. He stopped, but the look of shock on his face was priceless.
Still never stopped him from making snide remarks about others size as though that was a measure of the value and character.
Caught him on it once again in a gathering including non-family members when he was making remarks of a cousin's recent second husband's size (I've never met) who is tall and hefty...at the time she married him. She's always been a very short, normal weight person.
Her first husband was normal body weight when she married, but also an alcoholic who developed the beer gut.
Anyway, I said to my dad, "So, does being fat make him a bad person?"
Again, shocked into silence.
Remember, these comments are not about us/the fat person, but the poor character of the person who makes the idiotic remarks.