My vet, and his helper, are both boxer owners. They tell me that boxers grow quickly, however, they don't really fill out until they are 1 1/2 to 2 years old.
My boxers tend to go through stages. They seem to fill out, then look slightly skinny again...etc... . My grown children say they (the boxers) are too skinny, but the vet thinks they are fine. I would rather them be slim, and perhaps not become obese once they start to settle down into real adulthood. There is a lot of information on Boxerworld regarding boxers that tend to be thin while they are young.
Delilah is pretty small too. I do not think she has topped the 50 pound mark yet, at 12 months old. She eats a lot more than Samson, who is at least 65 (perhaps more now) pounds. Please do not fret about your boxer's weight. As long as they are healthy, eating well, drinking water, and happy, I think we can be assured that they will fill out!