I just had a full steak dinner with beans and potatoes, then afterwords I ate 3 cups of nuts, an apple, and 2 protein bars. I ate because I still felt hungry. Why was I still hungry?

You were probably very hungry and maybe having a growth spurt. Don't worry about it. You won't be able to do that very often without getting fat, especially as you get older, but you won't want to either. Eating that many calories is a lot of work.

At your age you most likely will not get fat eating like this. I would say long term you will get fat because your carbs are pretty high and your fat is low. Overall you are eating too much. Your appetite is because protein and carbs do little to tame your hunger. Your activity level will have little to no influence on you long term. Regardless of activity the majority of people with western [high carb low fat] diets steadily balloon after 30.

I've recently come across articles related to "leptin resistance" and how for some people certain types of carbs cause the level of hunger to increase, instead of decrease. I can say for myself that if I have very few carbs in a meal, i feel less hungry afterwards, even if I ate less. http://www.coachcalorie.com/leptin-resistance/

Calories can indicate how much weight one can gain only when all other variables stay static. Since all other variables are currently unknown and in flux, none of us would have any idea.

That doesn't sound like 1500 calories, it has got to be way more than that. As long as you don't eat like that all the time and stay active, you will not get fat.

You most likely had over 4000 calories, and if that's all you eat for a day an a half, no, you will not get fat. If you eat like that every day, you will get fat. Why you were still hungry after all this food is because something is wrong. Either you eat because eating comforts you emotionally, or something is wrong with the system that governs hunger.  If you are already obese, there is a good chance that your hunger/satiety system is already compromised with Insulin and :eptin resistance..  .

If you eat like this on a daily basis,yes you will get fat.When you do your workouts,you also need to watch your diet.If you don't watch your diet then your workouts are wasted.