Wow! I don't think she's small.

Jasper weighed 43(I think) pounds just 2-3 weeks ago at the vet and hes 8 months old! He used to be 45 and I expected him to be almost 50 or more last time, but he lost weight, as he doesn't like the food very much, but he has to eat it for now, until we find out what he's alergic to
I think he's going to be alot smaller than alot of boxers.

Also, boxers are a slim breed, I don't see many fat ones! I think the ribs being visible is common in them:

You can see Jasper's ribs in this picture, but not as much, it depends on the posistion hes in, sometimes you can almost count them all by looking at him