Losing weight eating a lot?

I count all my calories and am getting between 3400-3800 calories a day and am still losing and or maintaining weight. When I try to eat less then 3000 I start losing weight very fast. I go to the gym only twice a week, run hard and walk on average about 3-5 miles a day. Is that really enough to need to eat that much?

I don't eat very healthy. Muffins, danishes, sweet rolls or pie for breakfast, a king sized lunch and a big dinner usually. Fast metabolism or tape worm?