I am increasing my pectoral strength, but not my size, what gives?

I began a weight lifting program 6 weeks ago. I have gone from 30 pound dumb bell presses on a flat bench to 55 pound dumb bell presses on a flat bench in that time period. However, I have noticed NO size gain whatsoever, just strength.

Am I not eating enough protein? What should I be doing? Is 6 weeks not enough time to notice tremendous growth?

Note: I am on a 1300 calorie/day diet because I am trying to lose 90 pounds by August 1st, so I am not thin. Also, I am doing other chest exercises such as incline dumb bell lift.