Due to childhood malnutrition, I was always very skinny. I was about 35 odd pounds at seven years old, to give an example. When I started high school I was about 105. When I left home, I was probably 120 or less--didn't weigh myself, but I think it was less.
I lived under conditions of privation for a while. Finally, around 2006, I made it all the weigh (ha ha) to 160. An extremely unaccustomed weight for me.
Now, in 2020, I'm probably about 200. Almost a 100 pound gain in 20 years. I know it took a while, but I got there. I have a tummy now. I'ts still rather small but it makes putting on shoes and socks a little difficult. My legs spread involuntarily when I sit down. I still wear about a thirty-six inch waste but it's a bit tight. I can feel my belly touch my thigh when I lean over a bit. I wear a 2x shirt, when I used to be able to wear child sizes even as a twenty-year-old lol. What a difference.
That was probably the "ah-ha, i'm chubby" moment. When I had to get a 2x shirt for the first time in my life.