Boxers all grow at different rates. They have a few growth spurts where them may tower over other pups their same age, and then may seem tiny a few weeks later. They will go through akward stages where their legs are too long and their head too small. LOL. They are like grow kids.
With these different growth rates, it is difficult to tell if they will so down or keep growing fast. However, boxers do not stop growing upwards until around 18 months of age and then fill out until around 3 years. So if a 6 month female weighs around 50 lbs, I think it is safe to say she will be on the larger side.
Anyways, I'd love to hear more about your adorable wiggle-butts!