Michael Phelps eats 12000 calories a day and has abs many people would kill for.
What does this mean?
It all comes down to calorie usage versus intake. If your body only uses 1000 calories but you eat 2000 a day, you're going to gain weight, vice versa and you'll starve to death (that's how many people have died in various prison camps).
For me, I realized that I feel tired and can sleep a lot more when I don't get enough calories. I even had that weird fatigue feeling I get when I'm fasting for a while before I realized what was going on. It's really strange eating normal sized meals and feeling like that.
If I eat too much, I usually have gas and feel "gunky" inside.
But i'm sure that you'll come up with your own way of telling. If you're starting a routine, It's okay to eat as much as you were before (or maybe even a LITTLE less if you were even eating too much). But be sure to listen to your body, it knows what's best for you and will tell you what's going on. Chances are that after a while your resting calorie usage will increase and you'll lose enough weight from the exercise that you won't have anything extra to burn if you were breaking even without exercise. When you hit that point, you'll need to start eating more.
I don't encourage losing weight by cutting calorie intake alone. I did that a lot before my mission and I think that it caused my body to disintegrate to a large extent. Exercise is really healthy and you'll feel better too.