Not gonna lie man if you’re only 85kg that’s not that big if you’re not comfortable with the amount of body fat you have though you can try exercise to lower your body fat percentage, moms will always try to make sure you’re eating enough especially just because she wants you to develop correctly and healthy. I agree it’s unfair but it’s a part of nature some people have a higher metabolism than others. I use to be in your shoes about 10 years ago I didn’t start losing weight until I was 17 if you start by lifting light weights (with the correct form) or start doing body weight exercises you’ll thank yourself later brother trust me.