You could increase your exercise, but the sad fact is that the smaller you get, the fewer calories you burn. I am a short person to begin with, and sometimes I feel like it's just not fair. I maintain at the amount many people lose at. My best advice is to do some strength training so you can build muscle tissue, which burns more calories at rest, to a degree.
300 seems like a lot for a 15 pound loss, though. Have you tried plugging numbers into a different online calories burned calculator?
And now that you don't have to be perfect, you can be good.