> It's volume. I eat too much.

Bingo. I've had this conversation with my girlfriend. She's not overweight by any means (160 cm ~55 kg) but I am quite underweight (182 cm, 65 kg, < 10% body fat) and the conclusion I've arrived at regarding our differences in body composition is because she routinely eats 1.3x to 1.5x the amount I eat. We both exercise and eat little junk food.

I'm convinced it has to do with upbringing. My family never ate a lot and the portions I was served as a kid weren't big. Nobody in my family is anything more than skinny. Her family however, they eat a lot (and healthy too, mind you). This is all influenced by other factors, her parents and grandparents grew up with not a lot to their names and with food scarcity, so when they reached a position in life where they could comfortably afford to eat they gained the habit of having big meals. My family was mostly more privileged in that regard so maybe they never felt the need to focus on food as much.