I suspect that is because your metabolism has been trained to burn a lot faster than most.
When I was under 30, it didn't matter how much I ate, I could burn off everything! I probably averaged 4,000 calories a day. I weighed about 185ish. Now I weigh about 172-73. I know muscle will burn more than fat, so it's good to have muscle to help keep your weight down.
I always laugh when people say they don't want to lift weights because they're afraid they'll get too big. That doesn't even happen to some people that are trying to "get big".