Net calories on heavy calorie burn days

I find it very hard to eat enough calories on heavy workout days. I have been using HRM to track calorie burn and think it is reasonably accurate but some days I burn 700-1,500 calories and my net is often under 1,000 calories. I feel good but I wonder if I need to consume more calories on these days or if because I feel fine then I probably am. Recently have been working out more and as a result have several days and even weeks well under calorie goal but not sure if this is good or not. Any thoughts?