My son (6m) keeps calling me (32f) fat whenever he spends time with my sister (26f)

My son is 6 and adores his aunt (my sister) and my sister is a good person.

I’ve always been bigger than her. For context, I’m 5’0, roughly 150-160 lbs, havent weighed myself in awhile and my sister is 5’3 and 115 ish lbs. She’s always been very skinny and works out. I think its genetic that she is thin since she also consumes a lot more than me and tends to eat more junk food than me.

I used to be insecure but my sister always assured me I am perfect the way I am and always calls out fatphobia. However, when my son started to talk more and hang out with her more, he comes home to me and asks me questions like: “Why are you so much fatter than April (my sister)?” “Youre so much shorter and wider than April.” “April eats more but is skinny. Why?” He really hurt my feelings when he pulled out a pair of my pants and said “12! 12! Double digits! Thats huge!” I know it sounds kinda funny but Im a sensitive woman lol

I try to explain to him every body is different and stores weight differently but the questions keep coming and getting repeated. Even when I set my foot down and tell him to stop, he will continue once he sees April again.

He’s also making observations towards strangers. He asked me in a crowded shop why “Theres a ton of fat people here but not where April lives.” For context, April is my adopted sister and she’s originally from Asia so my son is talking about how Americans are fatter than Asians.

I really have tried to talk to him and explain to him why what he is saying can be hurtful and try to answer honestly but hes super fixated on this. I also doubt April is putting these thoughts in his head on purpose as I know her to be defensive and respectful about people’s bodies.

Im just not sure what to do now or if I just gotta wait out this phase.

tl;dr: son compares me to my thin sister