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?
"If you drop an egg, you don't say, 'Oh, shoot' and drop the other 11, do you?"
-Source unknown, but obviously brilliant
Reached goal 4/16/2010...but kind of afraid to look these days