I'm putting it down to eating less. Regardless of what you think you're eating the reality is that your metabolism and digestion (gut microbiome or whatever) is nothing special. If you actually measure and weigh everything you consume, you'll find the total calories are in the normal range for someone your size.
If you think that there's something else going on, then let's see some hard data.
CICO is the mechanical answer. The question OP is getting at is "why" they naturally eat less. The poster is saying "I eat until I'm full and don't gain weight". For other people, they eat until they're full and gain weight.
Personally, my "satiated" level is very low. When trying to gain weight while weightlifting, it was difficult to eat the amount of calories needed. I know others who can easily eat twice as much as me in a sitting and not feel full. Something different in happening in our bodies to signal "stop".
If you think that there's something else going on, then let's see some hard data.