So, what is the scientific reason I cannot eat as much as I did, 30 years ago?

When I was a teen, I could get up in the morning and cook up 7 to 10 pancakes, and scarf them bad boys down and not feel sated. When I was in college I could eat a entire deep dish personal pizza (with a little effort to finish), buffets where my friend. These days I eat two pancakes with some eggs and I am almost full. I get two McDoubles thinking I will have both and then get full after one. So, what is the scientific reason why I don’t or can’t fit as much food in my body as I use to, when I weight 26lb more than I was then?